INTRODUCTION

X-pinch is a broadband radiation source. At the same time, different X-pinch regions have different radiation parameters: the size of the emitting region, the duration of emission, and the radiation wavelength. The X-pinch hot-spot radiation corresponds mainly to the region of soft X-ray radiation (0.2−2 nm) [1]. After a hot spot is formed, an electron beam with a shorter wavelength component (<0.2 nm) is generated. As the current passes, radiation is generated in the ultraviolet region (>2 nm) [2]. At the same time, any of these radiations can find various scientific and technological applications [3–5].

Although X-pinch radiation was studied for many years, precise determination of the energy emitted by various X-pinch regions is a difficult task. Since the X‑pinch is a sufficiently bright radiation source, the radiation dose can be measured and then converted into emitted energy using thermoluminescent detectors (TLDs) based on lithium fluorides LiF(Mg, Ti) [6–8]. After the first experiments, it became clear that the sensitivity of such detectors was sufficient to detect radiation with spatial resolution.

It is important to note that TLDs based on LiF(Mg, Ti) are immune to electromagnetic interference, are characterized by a linear response in a wide range of absorbed doses (from 20 mSv to 10 Sv), and are actually insensitive to UV radiation generated in a high-current pulsed electric discharge [6–9].

EXPERIMENTAL TECHNIQUE AND RESULTS

Experimental investigations of the X-ray intensity and the energy component of a hot spot upon a high-current pulsed electric discharge were carried out at the BIN facility (hybrid X-pinch with a 30-µm-diameter 2.3-mm-long Mo wire, a maximum current of 270 kA, a current rise time of 100 ns, and a voltage of 300 kV) at the Lebedev Physical Institute of the Russian Academy of Sciences [1]. The diagram of the BIN facility is shown in Fig. 1.

Fig. 1.
figure 1

Diagram of the BIN facility: (1) Marx generator, (2) intermediate capacitor, (3) switchboard, (4) forming line, (5) output spark gap, (6) vacuum chamber, and (7) X-pinch.

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Detectors of different types were housed in a vacuum chamber for diagnostics: probe detectors and the Rogowski loop were used to measure the current, and photo conducting detectors (PCDs) were used to measure the time distribution of the X-pinch radiation intensity (calibrated photo-conducting diamond sensors) [10].

Spatially resolved radiation detection was performed using a device for obtaining one-dimensional spatial resolution, i.e., a 50-µm-thick Ta plate with a 200-µm-wide 2-mm-long slit. The 50-µm-thick Pb shield was used to protect against background illumination. The distances from the slit to the X-pinch and to the photographic plate were 7 and 28 cm, respectively. The distances were selected so that the hot-spot radiation was incident on a single TLD. In this case, the hot-spot formation area was ~200 µm, and the detector size was 1 mm; therefore, the spatial magnification factor was five. The diagram of imaging is shown in Fig. 2.

Fig. 2.
figure 2

Layout of the experiment on spatially resolved detection of hybrid X-pinch radiation using a TLD based on LiF(Mg, Ti): (1) vertically positioned hybrid X-pinch, (2) horizontal slit with a width of 200 µm, (3) vertical array of TLDs based on LiF(Mg, Ti), (4) Fuji BAS TR photographic plate, and (5) PCD.

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In order to obtain the radiation distribution along the hybrid X-pinch wire, the slit was located perpendicularly to the wire direction. A TLD array was placed behind the plate so that the detector ends were positioned towards radiation to increase the spatial resolution. X rays from the hot spot were incident normally to the OXY plane (see Fig. 2) on the TLD array such that the plasma-spot image was at the center of the array (two TLD arrays were used in the experiment). X rays were also detected by a Fuji BAS TR photographic plate, which was sensitive to both UV light and soft X rays [11]. PCDs were used for time-resolving radiation detection in each shot; they were placed behind Al and Ti filters with a thickness of 4 and 12.5 µm, respectively. The PCD signals and the X‑pinch current are shown in Fig. 3.

Fig. 3.
figure 3

(1) Current signals through the X-pinch and the PCD signals behind the filters made of (2) 4-µm-thick Al and (3) 12.5‑µm-thick Ti.

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The signal from the photodetector behind the 12.5‑µm-thick Ti filter demonstrated the presence of soft X rays with wavelengths of 0.25−0.50 nm, which indicated the formation of a hot spot [11, 12]. At the same time, the PCD with the 4-µm-thick Al filter detected radiation with a wavelength of less than 1.8 nm, which provided a wider radiation bandwidth. This detector showed the presence of X rays before and after the emission of the hot spot. This indicated that there were other radiation sources, except for the X‑pinch hot spot, in this shot.

Figure 4 shows an image produced on a photographic plate by hybrid X-pinch radiation that passed through the slit. By changing the contrast of the image, it is possible to reconstruct the position of each TLD (two TLD arrays, 1 and 2, were used in the experiment; see Fig. 4).

Fig. 4.
figure 4

Image on a photographic plate with a visible TLD shadow, which was obtained during explosion of a hybrid X-pinch with a 30-µm Mo wire. The position of the TLD array was reconstructed by the shadow: (1) the first and (2) second arrays and (3) region used to construct the absorbed-energy profile.

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The profile of the radiation intensity distributions recorded by the photographic plate is shown in Fig. 5a. The profile was constructed along the OY axis in the region indicated by position 3 in Fig. 4. Two regions are discernible on the blackening profile, which have the highest intensity and the smallest radiating area. The area can be estimated based on the blurring of the edge of the slit image. These regions correspond to the hot-spot position in the X-pinch [11, 12]. Radiation from two hot spots was detected in this experiment. One of these spots was more intense, as indicated by the photodetector signals. In order to compare the absorbed radiation energy measured by each TLD, the integral of the blackening was calculated in the region covered by each of the detectors (Fig. 5b, curve 3). The distribution of the radiation energy absorbed by the TLDs is shown in Fig. 5b (curves 1, 2).

Fig. 5.
figure 5

(a) Blackening profile of the photographic plate and (b) absorbed energy detected using (1) the first and (2) second TLD arrays and (3) total detected radiation energy obtained in a region similar to the TLD location.

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The graphs show a good correspondence of the curves if we compare the energy absorbed by the TLDs (in relative units) and the blackening integral, which is proportional to the absorbed energy. This makes it possible to use the TLD to obtain the distribution of absorbed doses using a slit (or a pinhole camera) to construct a hot-spot image on a TLD array. If photographic plates are used, determining the absolute value of the absorbed radiation energy is difficult.

TLDs allow us to calculate the energy emitted from a specific region of a plasma object. In this case, TLDs 5 and 9 detected radiation from the hot spot. Calculation of the absorbed dose and conversion of the absorbed dose into the radiation energy allowed us to determine the energy that acted on the detector. The radiation energy was 69 and 94 µJ for each of the hot spots. Taking the radiation isotropy into account, the total energy emitted in all directions was 11 and 15 mJ for each of the two hot spots. The energy emitted from the hot spot can also be determined by the PCD signal as an integral of the measured radiation curve. In this shot, the radiated energy was 9 and 13 mJ for each of the hot spots. However, since the PCD was placed behind the Al filter with a thickness of 4 µm, its effective spectral sensitivity turned out to be lower; therefore, the resulting value of the radiated energy was feebly lower relative to the TLD.

CONCLUSIONS

The applicability of TLDs in spatially resolved measurements has been shown, which makes it possible to determine the energy emitted from specific areas of a plasma object. In the case of complex objects, such as an X-pinch, this makes it possible, e.g., to determine the energy emitted from various plasma objects, in particular, from a very interesting object such as the X-pinch hot spot, which is the most widely used source of soft X rays. The emitted energy was determined in a series of experiments; its values were 11 and 15 mJ for each of the hot spots formed upon X‑pinch explosion.

Although the TLD is large (of the order of 1 mm in the minimum cross section), its high sensitivity provides radiation detection with a spatial resolution of approximately 200 µm using a high-magnification scheme. In this series of experiments, the radiation energy was independently determined using the TLDs and the PCD. The experimental results agreed within at least 80%. Taking into account that the spectral sensitivities of the TLDs and PCD differ, we consider that the accuracy of the new method for measuring the radiation energy is high.

Experiments show that the TLDs can be used to good effect in diagnostics of plasma objects.