The Relativistic Proton Spectrometer (RPS) for the Radiation Belt Storm Probes Mission
The Relativistic Proton Spectrometer (RPS) on the Radiation Belt Storm Probes spacecraft is a particle spectrometer designed to measure the flux, angular distribution, and energy spectrum of protons from ∼60 MeV to ∼2000 MeV. RPS will investigate decades-old questions about the inner Van Allen belt proton environment: a nearby region of space that is relatively unexplored because of the hazards of spacecraft operation there and the difficulties in obtaining accurate proton measurements in an intense penetrating background. RPS is designed to provide the accuracy needed to answer questions about the sources and losses of the inner belt protons and to obtain the measurements required for the next-generation models of trapped protons in the magnetosphere. In addition to detailed information for individual protons, RPS features count rates at a 1-second timescale, internal radiation dosimetry, and information about electrostatic discharge events on the RBSP spacecraft that together will provide new information about space environmental hazards in the Earth’s magnetosphere.
KeywordsRBSP Inner radiation belt CRAND Trapped protons Solar energetic protons Total ionizing dose Relativistic protons
Analog Signal Processing
Consultative Committee for Space Data Systems
Cherenkov subsystem within RPS
Cosmic Ray Albedo Neutron Decay
Cosmic Ray Telescope for the Effects of Radiation
Combined Release and Radiation Effects Satellite
CRRES electrons (model)
CRRES protons (model)
Digital Processing and Interface Electronics
Geometry and Tracking (4)
High Voltage Power Supply
International Solar-Terrestrial Physics
Johns Hopkins University/Applied Physics Laboratory
Low Earth Orbit
Lunar Reconnaissance Orbiter
Low Voltage Power Supply
Micro-Channel Plate/Photo-Multiplier Tube
Mission Operations Center
National Aeronautics and Space Administration
Naval Research Laboratory Mass Spectrometer Incoherent Scatter Radar
National Reconnaissance Office
Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics
Proton Electron Telescope
Pulse Per Second
Panel on Radiation Belt Environment Modeling
Radiation Belt Storm Probes
Relativistic Proton Spectrometer
Solar, Anomalous, and Magnetospheric Explorer
Science Operations Center
Solid State Detector Assembly
Tri-University Meson Facility
1 Scientific Goals
Within two Earth radii of the Earth’s surface there is an unexplored region of trapped particles that are a challenge to accurately measure, a challenge to engineer for spacecraft design, and a challenge to understand given particle lifetimes that can exceed decades or even centuries. The inner Van Allen belt is a nearby reservoir of charged particles from a multitude of sources. We know the existence of trapped protons with energies beyond 1 GeV, secondary particles such as positrons, electrons, and light ions 4He, 3He, and 2H, electrons that diffuse inward from the outer magnetosphere, and heavy ions that originate as interstellar neutral particles. These particles execute their gyration, bounce, and drift motions in a region with a relatively strong and stable magnetic field that shelters them from transients in the geomagnetic field, yielding the longest trapping lifetimes for particles in the Earth’s magnetosphere. The tenuous upper atmosphere is both a source and sink of these populations, whose influence varies over the solar cycle and episodically during times of rapid atmospheric joule heating.
Sorting out the various sources and losses in the Inner belt is challenging but there are particle characteristics that aid the process such as: extremely high proton energies that distinguish these particles as having a galactic cosmic ray origin; heavy ion composition that identifies these as being anomalous cosmic rays whose access to the inner belt is only afforded by their being mostly singly-ionized; and antimatter composition that identifies another branch the products of nuclear collisions between galactic cosmic rays and atmospheric atoms. For electrons the picture is less clear since there is little information about the electron energy spectrum and no identifiable characteristics of an electron that originates from a nuclear interaction versus one that diffused inwards from the outer Van Allen belt.
This paper describes the Relativistic Proton Spectrometer (RPS) whose purpose is to answer scientific and applied questions about the inner Van Allen belt proton population. RPS is government furnished equipment for the RBSP mission. As part of the scientific payload, RPS will measure protons in the energy range of ∼60 to 2000 MeV with good energy resolution and with a design that accommodates the challenges of measuring a foreground proton population with a large background due to penetrating protons. It is strictly a proton spectrometer focused on the primary constituent of the inner belt. RPS complements the other energetic proton measurements on RBSP (Baker et al. 2012; Blake et al. 2012) by extending the proton capability of the mission into the GeV range beyond previous investigations from the near-geosynchronous transfer orbit of CRRES (Johnson and Kierein 1992).
The measurement difficulties in a high-background environment of penetrating particles, along with the mission challenges of operating in the inner belt, have allowed us to obtain only glimpses of the entire inner belt proton population, either at the bottom of the field lines in low-Earth orbit (e.g. Baker et al. 1993) or only up to ∼100 MeV at higher altitudes (Albert and Ginet 1998). By sampling the protons near where their intensities are greatest and by monitoring outer trapping boundaries on two RBSP vehicles, RPS will provide new insights into this unexplored territory of the inner Van Allen belt.
1.1 Inner Belt Origins
The RPS instrument objectives stem from a modern interest in the dominant inner Van Allen belt proton component. One can trace the RPS investigation to the beginning of the modern space age with the flight of Explorer-1 (Van Allen 1960). There are several compelling historical accounts of the first Explorer missions and the concurrent hypotheses about what later investigations determined were high-energy protons stably trapped in the inner belt (e.g. Chudakov and Gortchakov 1959). Reviews such as Hess (1963, 1968) provide a glimpse of these early measurements and the open questions, some of which remain unanswered today. Ludwig (2011) relates a more recent description of those path-finding space discoveries. An early hypothesis held that the trapped radiation was related to the aurorae, while others found a natural link to albedo particles formed from nuclear interactions of galactic cosmic rays with the upper atmosphere (Singer 1958). The relevant facts to remember here are: the trapped inner belt proton environment saturated the Explorer-1 measurement in 1958 with high penetrating rates; and there was an early appreciation for the stability of the inner magnetosphere to form a strong magnetic bottle for natural and artificial injections of high-energy particles.
1.2 Highlights of Inner Belt Measurements
We highlight a few studies of inner belt protons from ∼10 to ∼100 MeV. One quickly finds that the research interest in the inner belt peaked decades ago with only a few successful attempts at quantifying the inner belt proton spectrum over the past 20 years. Sawyer and Vette (1976) listed the missions that operated in the inner belt including observations from Explorer 4 in 1958 through the Azur mission in 1970. While the examples we highlight here reinforce the existence of a high-energy proton population, the next steps in quantifying the details of that population will require the RBSP mission. What was the case decades ago still obtains today: few satellites have spent significant time near the magnetic equator and at the peak intensities of the inner belt. These research examples added to our motivation to settle the unknowns of the proton energy spectrum: its shape, maximum energy, and time dependence.
The 1971-067 mission was a US Air Force Space Test Program mission with 8 space vehicles. The propulsion module called OV1-20 included a Cherenkov counter telescope that orbited for ∼21 days in a 130 km by 1950 km polar orbit. The counter itself operated for about 8 days in August 1971 using an on-board battery for power. A pair of thin scintillators defined the geometry and a Lucite Cherenkov radiator yielded an energy range of ∼65 to >550 MeV (Kolasinski 2012). The limited OV1-20 measurements were not consistent with the AP-8 model of the inner belt (see the model discussion below). There were plans for follow-on missions to measure the inner belt for longer time periods, although those missions did not occur.
RBSP will be the first mission since the Combined Radiation and Release Effects Satellite (Vampola 1992; Johnson and Kierein 1992) to survey the inner belt using a near-equatorial orbit and suites of particle and fields experiments. Although the CRRES orbit itself was sufficient for investigations of the inner belt, the vehicle did not have instruments to measure protons above ∼100 MeV and many relevant CRRES instruments did not perform well in the inner belt because of large backgrounds from penetrating protons (e.g. Gussenhoven et al. 1993).
After the end of the CRRES mission 1992, the only other relevant science data originated from low-Earth orbit (LEO). Continuing LEO observations, though important for situational awareness and refinement of our understanding of atmospheric losses, cannot obtain the information we need to validate our understanding of the inner belt proton sources and losses. The steep flux gradients caused by the atmosphere introduce large uncertainty in the modeling (e.g. Heynderickxs et al. 1999).
The SAMPEX mission provided a long baseline of inner belt proton measurements below ∼600 km (Baker et al. 1993). The PET instrument was a cosmic-ray sensor design that monitored the outer belt electron population in LEO, solar energetic particle precipitation over the polar caps, and the trapped protons in the inner belt. Figure 2 shows changes in the outer boundary of the trapping region from SAMPEX/PET.
The PAMELA magnetic spectrometer is another example of obtaining information about the inner belt from a cosmic ray detector placed in low-Earth orbit, in this case an elliptical 350–650 km orbit (Adriani et al. 2008). PAMELA benefits from a large geometry factor and an extensive energy range designed to address galactic cosmic ray antiparticle abundances. Similar to SAMPEX, their measurements indicate an inner belt energy spectrum above ∼100 MeV that is harder than the AP8 model discussed below.
1.3 Inner Belt Proton Models
Models of the inner belt protons have been developed to predict the penetrating backgrounds in science and engineering instrumentation, for science analysis, and for spacecraft engineering. We briefly discuss them here because they represent an efficient way of comparing new measurements to prior data sets and because we designed RPS to meet the needs of future models.
A host of LEO models address the low-altitude fringes of the Inner Belt, but only two sets of specification models describe the heart of the Inner Belt environment: AE-8/AP-8 (Vette 1991; Sawyer and Vette 1976) and models from the CRRES mission (Brautigham and Bell 1995; Meffert and Gussenhoven 1994).
From 1966 to 1976 there were a series of publications that compiled the available trapped proton (and electron) measurements in empirical models for applications and space science. The last versions called “AP-8” and “AE-8” for protons and electrons respectively are the de-facto standard for spacecraft engineering and are commonly used references for new measurements. We briefly discuss AP-8 and select a few inner belt proton studies since the creation of AP-8. See Ginet et al. (2012) for further details of the AP & AE models and their follow-ons.
The goal of the trapped proton and electron modeling effort by Vette and others was to synthesize a number of datasets with different coverage in energy and spatial location into a single reference model for engineering and research. Episodic updates using short-term datasets of that era, sometimes only a few months in duration, kept the models in step with the new measurements. There were 43 separate missions that contributed to the last version of the proton model, most of which provided little or no coverage of the inner belt.
The actual model took the form of least squares fits to integral energy spectra at various B and L coordinates. The authors drew smooth curves by hand through the data if there were too few points for a least squares fit, sometimes missing the data by a factor of 2 or more in order to constrain the fit to be smooth. The authors remarked that “the spectrum in the inner zone for energies above 150 MeV is not well supported by data in these models” and in fact above 150 MeV the model consists only of extrapolations of the least squares fits. They anticipated using the data from new missions to populate the higher energies but this addition never occurred. Another problem is that the mapping to B-L space for AP-8 took place in a static 1960 Jensen-Cain magnetic field model (Jensen and Cain 1962), limiting its usefulness today given the secular changes in the geomagnetic field.
Data from the CRRES spacecraft were the basis for the CRRESELE/CRRESPRO models. As mentioned above, the relevant instruments did not perform well in the inner belt. The proton datasets extend from 1.1 to 90.4 MeV and are therefore insufficient for questions relating to the exact shape and energy extent of the CRAND energy spectrum.
Heynderickxs et al. (1999) created an inner belt model for LEO using SAMPEX measurements that extended to 500 MeV and found that the SAMPEX data were higher than the AP-8 model by factors of ∼2 at several hundred MeV. The authors noted that the secular drift in the geomagnetic field means that data of different epochs such as SAMPEX and the 1970’s era AP-8 cannot be compared without transforming both data sets into the same magnetic field model.
1.4 Inner Belt Engineering Hazards
There have also been documented cases of induced radioactivity in materials. This is a potential problem for LEO spacecraft with astrophysics instrumentation that includes large volumes of scintillator material such as sodium iodide (Peterson 1965; Fowler et al. 1968; Fishman 1977). The effect is a tail on the background count rate that impacts the primary measurement for minutes after leaving the inner belt. The mechanism is photon and electron emission as the transmuted nuclei decay into stable elements. See O’Brien et al. (2006) for more details of these and other engineering concerns.
1.5 Primary RPS Science Questions
What is the energy spectrum of the inner belt protons?
How does the spectrum compare with the CRAND mechanism above 100 MeV?
What are the major sources for protons below ∼100 MeV?
Can we account for long-lifetime protons of ∼GeV energy using our current knowledge of the CRAND process and the secular variations of the geomagnetic field?
What are the causes of changes to the outermost limits of the proton-trapping region?
What determines whether an interplanetary shock forms a new proton radiation belt?
What geomagnetic activity determines how these transient-related belts are lost?
2 RPS Requirements
The prime concern for the RPS development is the clean separation of foreground events from the larger rate of penetrating background events. Some previous proton instruments have suffered from insufficient appreciation of the background in the inner belt, hence our need for an instrument that has the minimization of background as a fundamental part of its design. Other major concerns were sufficient margin for particle intensity, both to prevent instrument saturation and to insure sufficient statistics on the spatial scale of 0.1 L; accurate monitoring of instrument deadtime; and accurate in-flight calibration to monitor any changes in the instrument response. Our flux accuracy requirement applies when sufficiently high proton intensity is present so that the error from Poisson counting statistics is negligible and the remaining uncertainty in the flux originates from systematic uncertainties in the instrument itself. This will obtain over short (minutes) timescales in some regions of the RBSP coverage in magnetic coordinates, and over longer (several days) timescales in others.
Figure 5 also indicates the mapping from the higher-level instrument requirements to the detailed RPS attributes captured in level-3 requirements. The following section describes how we addressed these requirements.
3 RPS Instrument Description
3.1 Design Overview
A traditional method of discriminating a penetrating background from desired coincidence events in charged particle telescopes is to use an anticoincidence detector that surrounds the primary detecting elements (e.g. Krimigis et al. 1977). An anticoincidence cup would not work in the inner belt because its trigger rate would be much larger than the rate of valid events leading to a crippling deadtime. Another method is to discriminate against background in the time domain using fast timing coincidence to define valid events; the timing requires fast detectors such as scintillators and corrections for timing walk. The Cherenkov counter on OV1-20 used this timing method. The fast timing method does work in an inner belt environment; however it requires careful and potentially challenging coupling of the scintillator detectors to their light amplifiers and the latter will have gain changes in the environment if one uses traditional photomultiplier tubes, or there may be impacts from single event effects if one uses solid-state photon detectors.
In order to balance the interest in using standard charged particle detection techniques against a large instrument deadtime, the RPS concept requires a 10-fold coincidence in a stack of silicon solid-state detectors for a valid measurement. RPS is therefore not a standard range telescope because it does not measure the range of particles in a silicon stack; rather, we only analyze events that penetrate the entire silicon stack.
3.2 System Block Diagram
3.3 Solid-State Detector Assembly
The solid-state detector assembly (SSDA) consists of a total of twelve silicon solid-state detectors, their mechanical mountings, in-flight alpha sources (for the energy-sensing detectors), and a shielding housing to limit the singles rates due to penetrating protons and electrons. We procured the detectors from Micron Semiconductors, LTD. The technology uses ion-implanted doping to form a p+ junction on n-type silicon with nominal thickness of 1000±50 microns. The junction window is a continuous aluminum layer 0.1 micron thick; the ohmic window is continuous aluminum but 0.3 microns thick. Full depletion obtains at a nominal 70 to 90 volts for the flight RPS detectors; we employed a common bias voltage of 170 volts to insure full depletion of the silicon for all the flight detectors. Leakage currents are typically 100 to 200 nA.
We also made a provision for continuous alpha particle stimulus of the D detectors during instrument testing, integration, and in-flight, hence the 4-mm diameter extension of the active area outside the circular active area. We attached holders for the alpha sources to either side of the central aluminum detector mounts. The in-flight calibration section has more specifics on the in-flight alpha sources.
All the paired detector housings mount to a 3 mm thick Mallory base. The SSDA base attaches to an aluminum plate that is electrically and thermally coupled to the RPS chassis. A 3 mm thick Mallory cover fits over the remaining 3 sides of the SSDA providing electromagnetic shielding for the detectors and particle shielding corresponding to the range of 53 MeV protons and 5.3 MeV electrons; these approximate energies include the RPS mechanical housing of 350 mils aluminum.
3.4 Cherenkov Subsystem
The Cherenkov subsystem contains the Cherenkov radiator, its mechanical support, and the microchannel plate photomultiplier tube (MCP/PMT) for conversion of the Cherenkov light to an amplified electronic signal, and the bias network for the MCP/PMT. We refer to the entire subsystem as CRA. The radiator is optical-grade magnesium fluoride (MgF2). We chose MgF2 for its index of refraction of 1.38 which yields a threshold energy of ∼432 MeV for protons to generate light in the medium. The actual index of refraction depends on wavelength as described in the Geant4 modeling section below.
The conical shape of the radiator matches the field of view defined by the SSDA geometry. We suppressed internal reflections of light from backwards-going protons by painting the topmost surface of the radiator with a space qualified conducting black paint. A spring-loaded assembly holds the radiator in place; we used this method of pre-loading to mount detectors in the LRO/CRaTER instrument (Spence et al. 2010) and it proved itself useful for RPS. Here the primary concern was possible mechanical damage to the radiator crystal in the vibroacoustic launch environment.
An early design trade considered using a monolithic radiator and conventional PMT in order to eliminate any vacuum gaps or interfaces where internal reflections might cut the light intensity. However there was not sufficient time to pursue development for the RBSP mission, so we procured a UV-sensitive MCP/PMT assembly from Burle Industries. Their 85001 device incorporates a bialkali photocathode deposited onto the backside of a fused silica window; the nominal spectral response is 185 to 660 nm. The window area is 51 mm square. Inside the “tube” is a chevron pair of microchannel plates that amplify the photoelectrons and an anode with 4 equal area quadrants to collect the resulting charge cloud. We did not use the position information that the separate anodes provide, deciding instead to add the 4 quadrants together with a coupler printed circuit board since the prime measurement for RPS is the total light amplitude.
The Burle design allows for a smaller Cherenkov system than a conventional PMT would require, but there is a vacuum gap between the MgF2 radiator and the photocathode. Our early design simulations using the Geant4 tool indicated that there would be a negligible loss due to the gap for incident energies near ∼600 MeV, but that this loss would occur well below the 185 nm threshold of the Burle device. The result is that we capture the bulk of the light output from the radiator and the design is insensitive to cutoffs due to total internal reflections.
We potted the tube into an aluminum housing that also houses the resistor divider network that provides the correct bias voltages to the photocathode and to the MCPs. With 500 MeV protons we measured the flight model tube gains to be on the order of 105 at nominal high voltage of −1950 volts.
3.5 Analog Signal Processing
The Analog Signal Processing (ASP) subsystem accurately collects the pulse height amplitudes of detection events and provides in-flight electronic stimulus of each of the signal processing chains. There are a total of 13 ASP printed circuit boards in each RPS with one ASP dedicated to one detector element (12 for SSDA and 1 for CRA). Our analog processing follows the standard approaches of modern nuclear signal processing with these attributes: a hybrid charge sensitive preamplifier with a low-noise junction field-effect transistor; pole zero cancellation; active multi-pole shaping amplifier; a gated active baseline restorer; a gated peak stretcher; and a synchronous Wilkinson rundown for the pulse height conversion.
Another feature of the ASP system is the use of a dual-threshold detection. A low 50 keV threshold for the D detectors determines whether a deposit in a detector might be a candidate for pulse-height conversion. A higher threshold of 200 keV must then be triggered for actual pulse height analysis. We discuss the coincidence method more in Appendix A. The use of a lower threshold minimizes the walk in the timing coincidence window that is 260 nsec wide. The full range for energy loss measurement in the D detectors is 0.2 to 10 MeV with approximately 20 keV FWHM of system noise.
The ASPs for the D energy sensing detectors incorporate the full circuitry for the PHA conversion. Each valid deposit is digitized with the Wilkinson rundown circuitry in an Amptek PH300 hybrid with a maximum rundown time of 61.44 microseconds and 10 bit conversion, yielding a channel width of ∼10 keV. We perform the rundown simultaneously on all ASPs. Logic within the programmable gate array on the DPIE (next section) controls the PH300 devices.
The ASPs for the four A collimation detectors do not include the PHA circuitry. The single ASP that is dedicated to the CRA required that we attenuate the input by a factor of 18 in order to make the anode charge pulse amplitudes match the range of charge pulses seen from the SSDA, thus enabling all the critical components for signal amplification and processing to be duplicated across the physics package.
3.6 Digital Processing and Interface Electronics
The digital processing and interface electronics subsystem (DPIE) is the data processing unit for the multiple subsystems within RPS. DPIE is a single printed circuit board that contains the command and telemetry interface to the RBSP spacecraft, the power and logic interfaces to the ASP subsystem, and the main coincidence event processing logic. DPIE also creates the RPS CCSDS data packets that include pulse height and rate scaler data from the SSDA and CRA as well as all of the instrument voltage, current, temperature, and radiation dose monitoring.
The field programmable gate array contains the detector scalers, the coincidence logic, and the interfaces to the ASP system for the start of the rundown of valid events. The singles counters tally the number of low-threshold (50 keV) triggers in each active detector; there is a unique double coincidence counter for the selected pair of geometry-defining A detectors. When a valid coincidence event occurs (the nominal case is the 2 active A detectors and 8 D detectors), DPIE sends a signal to the ASPs to begin the rundown of stored charge in the PH300 PHA devices. The gate array latches the event time at the time of the coincidence, inserts the peak amplitudes and other information into the event packet, and transmits the packet to the RBSP spacecraft on a 1-second boundary. An internal counter monitors the time spent processing events and reports it every second as the total system deadtime.
DPIE processes all RPS commands received over the RBSP low-voltage differential signal command interface and uses the 1 pulse per second (PPS) interface for packet creation. RPS does not use any other instrument data or spacecraft housekeeping for autonomous operation, nor does RPS contain any software. One last feature worth noting here is that DPIE gate array monitors the command and PPS interfaces for fast (<120 nsec width) pulses that might be induced on these lines from electrostatic discharge (ESD) events somewhere on the space vehicle. RPS reports accumulated counts of these fast transients in its routine telemetry. Ground-based testing of RPS with a human-body-model ESD gun verified that the instrument had no anomalies from 5 kV pulses, and that the DPIE counted and rejected the induced pulses on the PPS and command interfaces.
3.7 Power Supplies
The RPS low voltage power supply (LVPS) is the power interface between the instrument and the RBSP spacecraft. A space-qualified EMI filter provides the first interface to the spacecraft LVDS power system, followed by three 5-watt DC to DC converters and other power conditioning and filtering to limit inrush currents on the power return and to satisfy the observatory electromagnetic cleanliness requirements. Typical efficiency for the LVPS is 55 %. The rear of the RPS mechanical structure houses the power supply on a single printed circuit board in its own cavity.
The high voltage power supply (HVPS) provides the SSDA bias and the higher voltage for the CRA on a single printed circuit board. The SSDA bias is 170 volts using a custom transformer and a two-stage Cockroft-Walton voltage multiplier. Note that all silicon detectors derive their bias from a single power supply. On the same board separate circuitry using a four-stage Cockroft-Walton series provides the −1650 volts to −2200 volts required for the CRA. A digital to analog converter on the DPIE provides the control signal for the MCP/PMT bias enabling us to compensate for changes in the MCP/PMT gain on the ground and in orbit with 0.14 volts per command step of 0 to 4095. When the HVPS is armed, then all outputs are nominally 2 volts; a disarm command sets the outputs to 0 volts.
3.8 Mechanical & Thermal Systems
The RPS mechanical housing conforms to the RBSP mission requirement of a minimum of 350 mils Aluminum shielding for internal components. Therefore the instrument is much stiffer than typical particle spectrometers with a fundamental resonance mode of the structure about 1.3 kHz. We designed the mechanical housing and its components around the main subsystems to allow for easy bench testing and integration of the engineering and flight subassemblies. For example, the SSDA is a self-contained mechanical housing that can be assembled and tested independently of the rest of the instrument.
We were concerned about the mounts for the Cherenkov radiator and the MCP/PMT because of the housing stiffness. For the former, we captured the radiator in an aluminum cone-shaped housing using 8 springs to pre-load the assembly. For the latter, analysis was of little use because of the complexity of the MCP/PMT stack. Phase-A vibration testing of a similar MCP/PMT led the manufacturer to stiffen the washers that hold the MCPs in place. Subsequent vibration testing on the flight models showed increased gains after shaking, which suggested some movement of the MCPs themselves might still be taking place with our design (see Appendix E for further details).
A single purge port allowed for purge during instrument testing and while on the RBSP spacecraft. A vent hole 0.1 inch in diameter vents the instrument via the LVPS cavity with a characteristic venting time of ∼1 second.
RPS is thermally isolated from the bottom deck of the RBSP spacecraft with insulating washers at each of the six mounting feet. A single thermal blanket, designed and provided by JHU/APL, isolates the bottom of RPS from radiation from the spacecraft. There is no direct solar loading of RPS since it sits on the anti-sunward side of the spacecraft. To control its temperature, RPS uses an operational heater located directly under the SSDA, a survival heater under the DPIE cavity, and a passive germanium black-kapton radiator on the space-facing top of RPS. Predicted temperatures for the SSDA on-orbit are between −20 to −10 degrees C.
One unique aspect of the RPS thermal subsystem was a thermal balance test that occurred before we built the flight model instruments. This earlier-than-usual balance test used a flight-like housing, circuit boards, and power dissipation. We performed the test to gain confidence in the thermal design and the thermal model before the flight model construction.
3.9 In-Flight Calibration System
RPS has internal calibration systems that enable us to monitor the SSDA and ASP performance. The first system is a commandable electronic pulser that sends precision amplitude pulses simultaneously to each of the 13 ASP boards equivalent up to 5 MeV deposit into the charge-sensitive amplifiers at a rate of 10 kHz. With the pulser we are able to monitor the 200 keV thresholds of the SSDA ASPs and simultaneously quantify the system noise by differentiating the curve of system rate versus input amplitude. The DPIE board houses the pulser; we can command the pulser on and off and adjust its amplitude with a 12-bit digital command level. Part of the routine on-orbit calibration will include stimulating the ASPs with the pulser as has been done with our ground-based functional test procedures.
We wanted a way to monitor the complete SSDA performance throughout the integration and testing of the instrument and observatories. As noted in Sect. 3.3, the detector design and mounts accommodate a holder for an alpha particle emitter to directly stimulate the 1 mm detectors. We procured a custom set of muti-nuclide alpha sources that contain approximately equal activities of 148 Gadolinium (3.18 MeV alpha, 74.6 year half life) and 244 Curium (5.76 MeV and 5.80 MeV alphas, 18.1 year half-life). The sources were unsealed and deposited onto 4 mm diameter titanium-platinum foils. The set of sources had an average activity of ∼3700 decays per second (90 nCi) which we scaled down with a small aperture in the source holder to yield approximately 8 events per second per detector in the flight models.
The alpha sources have been useful for monitoring the end-to-end performance of SSDA, ASP, and DPIE throughout the instrument environmental testing and will provide a useful benchmark for flight operations. It is important to note that only when RPS is commanded into its “alpha” operational mode do we pulse-height analyze the alpha events. The normal mode of operation sees no coincidences between any of the alpha triggers due to their low rates. Also note that the alpha sources will always be present in the quiet-time singles rates of the D detectors and will likely dominate those rates when RPS is not inside the inner belt.
Each RPS contains two micro dosimeters that measure the total ionizing dose and dose rate inside the RPS mechanical housing. These dosimeters are the second generation of the design first flown on board the Lunar Reconnaissance Orbiter (Mazur et al. 2011). For RPS, we telemeter the low and medium ranges that have 13.6 microRad and 895 milliRad resolution, respectively. We tested all four RPS dosimeters using a cobalt-60 gamma irradiator to confirm their dose equivalent gains, and have tested the design with 50 MeV protons (Mazur et al. 2011). The dosimeters accumulate the dose as long as RPS is powered on, requiring ground-based processing in the RPS Science Operations Center to accurately derive the dose rate and account for power-off intervals for the calculation of total mission dose. The dose rate and total dose information will be useful for monitoring the performance of RPS, of other systems on the RBSP vehicles, and for direct comparison with calculations of the dose derived from RPS and the other science measurements.
Within the DPIE cavity, one micro dosimeter has its silicon detector in the same plane as the SSDA detectors and facing the front RPS window, while the other is oriented 90 degrees away to explore for systematic pitch angle effects. To penetrate the RPS housing and micro dosimeter package and trigger the nominal 100 keV dosimeter thresholds it will require protons above ∼53 MeV and electrons above ∼5.3 MeV. The effective shielding is ∼3.25 g/cm2 for normal incidence, corresponding to an aluminum thickness of 474 mils.
3.11 Resource Summary
RPS instrument resources
Normal mode (no direct events)d
3.12 Geometry Factors
RPS geometry factor
Geometry factor (cm2 sr)
Threshold energy (MeV)d
Detector active area a,b
PEN events; bowtie analysis c
CHE events; bowtie analysis c
The goal of the RPS calibration is to develop quantitative procedures for converting the RPS data (rates, direct events, & housekeeping) into estimates of the proton energy spectrum and angular distribution. Calibration procedures consist of stimulating the instrument with electronic pulsers, particle sources, and particle beams. Calibration for RPS also consists of iterating the instrument Geant4 model to reproduce the particle data and to refine algorithms for extracting incident proton energy versus incident angle. Finally, in-flight calibration will use internal RPS stimulus and the modeled galactic cosmic ray proton energy spectrum. Hence, calibration for RPS will not stop after launch but is at a sufficiently high level of accuracy prior to the RBSP launch to allow for immediate RPS operations after instrument commissioning. This section briefly reviews the pre-launch and post-launch calibration techniques and their results.
The Level-2 requirements that drive the calibration work for RPS are those for energy resolution (50 % at 100 MeV) and for flux accuracy (≤50 % at 100 MeV). Together these two requirements impact almost every aspect of the RPS design, and we used all aspects of our calibration to verify that the sensors meet the requirements, and to monitor the constituent subsystems so that the instruments always satisfy these performance metrics. We meet the spin-plane angular resolution requirement of 10 degrees via mechanical accommodation on the RBSP vehicle, the RBSP MAG angular accuracy, and the 1/64 second fine time tags of the RPS direct events relative to the MAG data. Those uncertainties in angular response add to ∼4.3 degrees and are not expected to change with time.
4.1 Geant4 Model
Our algorithm for identification of the energy of incident protons after launch relies on a library of simulated responses of the sensor to well-characterized incident particles against which we compare the observed response of the sensor to real particles whose characteristics we wish to determine. We simulated RPS using the Geant4 (GEometry ANd Tracking) radiation-transport code version 9.4 (Agostinelli et al. 2003; Allison et al. 2006). Geant4 models the three-dimensional transport of individual particles of arbitrary species and energy through arbitrary material geometries, allowing the user to log information such as energy deposit and the generation of secondary particles along the trajectory of each primary or secondary particle.
Physics processes included in the Geant4 simulation of RPS
Relevant to these RPS subsystems
Electromagnetic energy loss
SSDA (primary measurement); CRA (scintillation); all (transport)
Includes scattering and dE/dx fluctuations for realistic straggling of range and energy deposit
Includes creation of knock-on electrons (delta rays) and bremsstrahlung photons that can carry energy away from detector volumes
Based on Geant4 Binary Cascade model
CRA; MCP faceplate
Photon spectral distributions are shaped by wavelength-dependent refractive index of optical materials
Spectral distribution from Viehman et al. (1975)
Optical photon transport
CRA; MCP faceplate
Includes wavelength-dependent absorption and partial and total internal reflection
Wavelength dependence from manufacturer specification
In order to ensure that our simulation represents the response of the actual instrument, ideally we would expose the real sensor to unidirectional, monoenergetic particle beams and compare the observations with models of the response. In practice, our primary proton calibrations used two proton beamlines at TRIUMF (Blackmore 2000). In the first, we used un-degraded 500 MeV and 350 MeV primary beams as well as the same beams degraded to lower energies with stacks of aluminum slabs up to 12″ thick. The other TRIUMF line provided 116 MeV primary beams degraded with Lucite up to 4 cm thick. In both cases a lead scatterer a few mm thick was also in the beamline upstream to lower the event rate. The beamlines included collimators and other inert material near the beam axis for focusing and intensity. This additional material resulted in proton spectra at the sensor that were broadened rather than monoenergetic. We therefore found it necessary to model the entire beamlines from the points where they entered the test area, placing the RPS sensor model shown above in the appropriate position. Thus the results we show below reflect the accuracy of not only the RPS model but also our models of the TRIUMF beamlines.
Our simulations had enough fidelity that, for example, we identified a contamination of ∼250 MeV and ∼450 MeV protons in the 500 MeV beam. Consultations with TRIUMF staff suggested non-nominal paths of some protons through the bulk of the collimators rather than through their apertures likely caused the observed lower-energy populations.
Figure 16 makes clear the presence of a CRA signal at proton energies well below the Cherenkov threshold. These photons result from proton-induced scintillation in the MgF2 radiator with an intensity of about 5 photons per MeV of energy deposit in our model. Viehmann et al. (1975) estimated a yield of about 20 photons per MeV for UV-grade MgF2. The lower scintillation yield for RPS suggests that the RPS radiator has a higher purity than the material that Viehmann et al. tested. We use the scintillation photon signal as an additional input to the algorithm that calculates the incident proton energy.
Lastly we note that the RPS development schedule prevented a calibration of the flight models at energies higher than 500 MeV. However, early on in the requirements development process we did calibrate an RPS prototype that included the same CRA assembly design as the flight models. There we were able to confirm the roll over of the CRA response, both actual and modeled, to 1 GeV.
4.2 Incident Energy Algorithm
Our process for deriving the incident proton energy from the RPS measurements relies on a pre-computed database of Geant4 test particle interactions with the RPS sensor. Using the modeled event rates as functions of incident energy and angles, we can accurately estimate the energy-angle response of the sensor. To estimate the energy of a single incident particle we constructed an algorithm (called “Enigma”) to perform a nearest-neighbors lookup against the database of modeled proton coincidence events. Specifically, we find a variable k nearest neighbors (k∼10) from the database in a 9-dimensional space defined by the energy deposits in each of D1 through D8 of SSDA and the number of photons detected by the CRA. From these nearest neighbors, we compute an average incident energy and an error metric. We implemented this computation scheme in the RPS Science Operations Center and hence all of what follows occurs in the ground-based processing of the data. We briefly discuss the steps in the Enigma algorithm below
Conversion from pulse heights to MeV for D1–D8 in SSDA is a necessary step to account for the slightly different gains of the ASPs and the few percent dependence on instrument temperature. Similarly, we convert the CRA pulse height to a count of photons using a gain factor established with the TRIUMF beam tests and the Geant4 simulations.
We transform each dimension of the 9-dimensional space (8 dimensions are MeV and the 9th is photons) onto [0,1] domains. The algorithm orders the values for each dimension using the Geant4 model event database: the smallest value maps to 1/(N+1), the largest value maps to N/(N+1), and the middle value maps to 0.5, where N is the number of samples in the database. If an observed event falls below the smallest value then the algorithm sets it to zero; events above the largest value get set to 1. The entire transformation follows the empirical cumulative distribution function (or percentile function) of the values in the database for each individual dimension. Therefore, distances in this transformed domain approximate differences in probability or percentile, given the assumed spectrum and angular distribution used to generate the database.
Using the 9 numbers from the observed event transformed onto the [0,1] domains, we find the k events in the database that are closest in response to the observed event. The algorithm applies a scalable weight to the distance in the D9 (photon) axis that is currently set to a factor of 20 to make the CRA response have the same weight as SSDA. The time it takes the algorithm to find the closest event in the Geant4 model database scales with log(N). Because k is much less than N, the set of neighbors is independent of the choice of the incident spectrum for the Geant4 particle simulations.
As noted above, we were able to inadvertently test the Enigma algorithm at the TRIUMF facility when the efforts to reduce the beam intensity produced near-monoenergetic contaminants in the 500 MeV beam at the level of a few percent of the primary beam intensity. We found the estimated contaminant energies to be consistent with material thicknesses upstream of the instrument.
We lastly note that the Enigma approach of using a nearest-neighbors look-up against a database of simulated events has applications to a broad class of particle sensors.
5 Flight Operations
We designed RPS to require minimal operations support. We rely on the extensive calibration efforts discussed above to produce RPS science in the Science Operations Center (SOC) instead of on-board processing of events. There is no on-board software. Choices such as these simplified the development, test, and integration of the flight models. We also recognize that we benefited from the targeted goal of only measuring inner belt protons. In this section we summarize the RPS telemetry, the main components and functions of the RPS SOC, and the primary data products.
5.1 Telemetry and Commands
RPS telemetry consists of two CCSDS packet types; Appendix B contains the details of both. The instrument produces a 304 bit long rate and housekeeping packet once per second whenever RPS is powered on and the instrument receives the 1 pulse per second signal; no command is necessary to start the flow of these packets. When integrated with RBSP, we have detailed information about all the RPS operations parameters, including SSDA and CRA rates as well as internal voltages, currents, and the micro dosimetry. Table 5 in Appendix B lists the contents of the rate and housekeeping packet. In addition to its use in RPS science, the RBSP spacecraft will include the entire RPS rate and housekeeping packet in its real-time space weather broadcast.
The second packet type contains the direct events which are the pulse heights for the valid coincidence events. Table 6 in Appendix B lists the contents of the direct event packet. Each event consists of nine 10-bit pulse heights and a 6-bit sub-second counter for more detailed timing analysis. The packet size is variable depending on the incident flux with up to 339 events per packet. RPS creates direct event packets when one or more valid coincidences occur in a second and when the on-board event quotas allow for transmission of the direct events to the RBSP spacecraft. The direct event packets follow the corresponding 1-second rate and housekeeping packet in the telemetry.
The ability of RPS to control its telemetry via event quotas was a simplification for on-orbit operations. A typical orbit will include two passes through the inner belt lasting 10 s of minutes, followed by quiet-time galactic cosmic rays above L∼2. RPS has a telemetry allocation of 2 kbps averaged across the orbit, and during quiet times we expect that RPS will not exceed this allocation. Of course, RBSP is in some aspects a discovery mission, so the exact nature of the inner belt proton intensity near the magnetic equator will not be known until after launch. However, during intense solar energetic particle events, it is likely that solar protons would exceed our orbital telemetry allocation. These will be scientifically interesting times to monitor the penetration of solar protons to low altitudes and to search for new radiation belts; we do not want to miss collections inside L=2 due to higher-altitude solar particles which will be measured with other sensors. Hence, we implemented 1-second quotas on the coincidence rates of events that trigger the SSDA and those that also have a valid CRA signal. These quotas are commandable in the range of 0 to 650 events per second for the sum of both SSDA and CRA events. The RBSP instrument telemetry data message format limits the size of a packet to 4086 bytes; the result is that RPS cannot send more than 339 direct events per second and quotas larger than 339 events per second will not telemeter more events. Routine operations will use the RBSP ephemeris predictions to estimate the times when each vehicle will cross an L shell of 2.0. On outbound passes at L=2.0 we will lower the event quotas to give priority to the direct events within the inner belt.
RPS has 36 distinct commands. RPS transmits a maximum of one RPS specific command following the spacecraft time and status information within the second. Appendix C lists the RPS commands. Two commands refer to the coincidence mode of the instrument, namely the normal mode in which a pair of A detectors defines the geometry and all D detectors are required for a valid event, and the alpha mode in which any trigger of a D detector creates a valid event. The latter mode is the only means of using the in-flight alpha emitters to create valid events. We can also enable or disable the in-flight pulser, enable and disable the high voltage, and set the CRA high voltage level. The remaining commands are for enabling and disabling individual detectors in SSDA, including the CRA, in order to remove any noisy detector from the nominal coincidence mode.
5.2 Science Operations Center
A governing concern in constructing the SOC was the need to keep the RPS science team aware of the RPS instrument states of health and the status of the SOC data processing. We added a series of scripts that routinely check the instrument status and the volume and contents of the various level-N data against tables and that announce anomalies or abnormalities via emails and text messages to pagers. Fault detection for RPS thus relies on the monitoring of housekeeping and performance in the SOC. Part of the warning message system includes a dead-man timer to insure that a human inspects the top-level functions of the SOC at least once every 2 days. The warnings scripts also automatically construct simple displays of the on-orbit proton flux compared with the AP-8 and -9 models for commissioning and early orbit operations.
5.3 Data Products
RPS data products
RPS CCSDS packets (decoded in CDF version)
By RBSP MOC
Nearly all L0 data; universal coordinated time (UTC), SSDA & CRA deposits; singles and coincidence rates, vehicle location, RPS boresight vector, magnetic field vector, estimated incident energy/angle, dead times (including quota effects)
L0 + 1–3 days
Daily up to L0 + 5 days, and on-demand
Mission elapsed time & UTC; flux versus energy spectrum (once per 5 degrees rotation); pitch-angle and full magnetic coordinates of RPS boresight in Olson-Pfitzer-Quiet and Tsyganenko models
L1 + 1–2 days
Daily up to L0 + 7 days and on-demand
Mission elapsed time & UTC; energy-pitch angle spectrum (once per spin and once per minute); full magnetic coordinates in Olson-Pfitzer-Quiet and Tsyganenko models
L2 + 1–2 days
Daily up to L0 + 10 days and on-demand
UTC; flux versus energy, pitch angle, and L shell; flux vs. other magnetic coordinates; phase-space density maps
L3 + 1–2 days
Daily up to L0 + 14 days and on-demand
We will perform the initial data processing with orbit, attitude, and universal time predictions and a model magnetic field. Reprocessing will occur as needed when we obtain the input parameters that global magnetic field models require and when the MOC publishes updates or revisions to attitude, time, and magnetic field vectors. We note that we can meet all the RPS science requirements entirely in the absence of external data because pseudo-static (e.g., Olson-Pfitzer quiet) magnetic field models are adequate organizers of the inner zone protons.
For science analysis, the SOC will routinely push the processed data to a location such as the NASA virtual radiation belt observatory. The daily data volume will be approximately 154 Mbytes for each RPS instrument. This yields an estimated data volume for the 60-day checkout plus a 2-year mission of approximately 300 Gb. The RPS SOC will also work with NASA personnel to transfer the RPS data to a long-term archive that NASA will provide.
The RPS instrument is focused on answering decades-old questions about the inner belt proton environment: the proton energy spectrum across a broad energy range and how it compares to the CRAND process, its maximum energy, and how it changes during geomagnetic activity. The observations from RBSP will foster new interest in this nearby yet relatively unexplored region of trapped particles and will raise new questions and insights about the processes that occur there.
We have implemented a robust instrument design that accommodates the lessons from previous measurements in this challenging penetrating background. RPS also incorporates in-flight calibration and subsystem diagnostics so that we can track its performance on-orbit and make any corrections its response. The RPS calibration and modeling efforts proceeded in pace with the instrument design and test, yielding a pre-flight model that meets our requirements for understanding the instrument performance and for producing useful science products immediately after on-orbit commissioning.
We thank Colonel C. Groves for architecting the PSBR program of which RPS is the space hardware component. R. Selesnick (AFRL) conceived of the RPS measurement concept. W. Olson managed the early phases of the PSBR project for the National Reconnaissance Office (NRO) and D. Byers (NRL/NRO) led PSBR at the NRO through flight model construction and delivery of the RPS flight models. C. Pritt managed the PSBR program office support for The Aerospace Corporation and B. Klatt provided quality assurance guidance for RPS. We thank T. Alsruhe for his contributions to the early project management and E. Blackmore of TRIUMF for his support of the RPS proton calibrations. We greatly appreciate the efforts of the NASA and JHU/APL RBSP mission partners for their cooperation, support, and participation in accommodating RPS on the RBSP spacecraft, specifically: K. Cooper, A. Reiter, D. Myers, J. Stratton, R. Fitzgerald, B. Williams, J. Troll, C. Herrman, N. Mosavi, D. Jones, A. Dolbow, and E. Rodberg (all at JHU/APL); S. Himes, S. Aloezos, and D. Sibeck (all at NASA/Goddard Space Flight Center); and D. Brewer and M. Kessel (NASA/Headquarters).
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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