Keywords

1 Introduction

Intense natural radiation is often observed in the sea-floor hydrothermal area due to radium and radon, which are originally produced by decay of uranium and thorium and are dissolved into the hydrothermal fluids. Measurements in the laboratory on land detect only long lived radioactive nuclei, whose half lives are in the order of years or larger, while immediate decrease of radioactivity of hydrothermal sulfide deposits is sometimes observed on the research vessel (Ishibashi, personal communications). The present study aimed to detect possible natural radioactive nuclei of short life time by on-board measurements with a NaI gamma ray spectrometer immediately after retrieving the sulfide deposit samples.

2 Samples and Experimental Procedures

Sulfide deposit samples were taken during NT11-20 and NT12-06 cruises with research vessel Natsushima operated by JAMSTEC in September, 2011 and in March, 2012, respectively. Four samples were examined as listed in Table 49.1, which were taken at sea-floor hydrothermal sites shown in Suppl. 49.1, and are shown in Fig. 49.1.

Table 49.1 Samples examined in the present study
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Fig. 49.1
figure 1

The samples examined in the present study

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As soon as a sample was retrieved, the sample block of about 15 cm in length was put into a plastic bag. It was then placed in front of the detector of a NaI gamma ray spectrometer having a 3″ detector, Aloka JSM-102, where the full scale of 240 channels are set up to 3,600 keV. The measurements with duration of 30 min were repeated several times successively or with some interval for each sample up to 90 h. After the cruises, the spectrum data were transferred to a PC and were converted to text files. The threshold method (Mercier and Falgueres 2007) was used to obtain the dose rates given by the samples. The counts from the 25th channel to 240th channel were integrated and compared with those obtained for standard calibration concrete blocks (Uchida et al. 2014) in order to obtain the dose rates.

3 Results and Discussions

The spectra observed in the first measurements of the samples are shown in Fig. 49.2. The peaks are identified as those due to 214Bi, 228Ac, 40K, and 208Tl. The spectra observed in the following successive measurements were essentially identical to those in the first measurements. 214Bi is a nucleus of uranium series while 228Ac and 208Tl of thorium series. These are most probably produced by decays of 226Ra and 228Ra contained in the sulfide deposits rather than those of 238U and 232Th as those concentrations in sulfide minerals are in ppm order while those in barite are in ppb order (Toyoda et al., 2014).

Fig. 49.2
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Gamma ray spectra of the samples measured just after the retrieval on the ship by an NaI gamma ray spectrometer

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The daughter nuclei of 226Ra and 228Ra were observed in HPD#1328G04, HPD#1329G05, and HPD#1331G05, but no daughter nuclei of 228Ra in HPD#1358R02a. This makes the most remarkable difference in the spectra shown in Fig. 49.2. As the half life of 228Ra is 5.75 years, the ages of the former three samples are estimated to be younger than several 10 years while the age of the last sample is to be more than several 10 years. The peak due to 40K is observed in HPD#1328G04 and in HPD#1331G05, but not seen in HPD#1329G05. As for the spectrum of HPD#1358R02a, it is not clear whether the peak around 1,461 keV is due to 40K or not. The latter two samples have much higher radioactivity due to daughter nuclei of 226Ra and/or 228Ra relative to that due to 40K than the former two samples probably because of the mineralogy which has not yet been analyzed. The Compton background of those and those daughter nuclei mask the peak of 40K in the former two samples. Except for 40K, the shapes of the gamma ray spectra of HPD#1328G04, HPD#1329G05, and HPD#1331G05 are very similar. The ratio of 214Bi at 610 keV to 208Tl at 2,614 keV is shown in Fig. 49.3, indicating that the relative ratio of 226Ra and 228Ra is the same for these three samples.

Fig. 49.3
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The ratio of the peak of 2,614 keV (208Tl) to 610 keV (214Bi)

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The dose rate given by the samples was calculated from the spectrum by the threshold technique (Mercier and Falgueres 2007) based on the measurements for U, Th, K calibration blocks with known amount of these elements with radioactive equilibrium. Although, therefore, the calculated dose rates should be slightly overestimated due to lack of the consideration for the nuclei between 238U and 226Ra, and 232Th and 228Ra in the measured samples, relative comparison would be meaningful.

The gamma ray spectra obtained by the repeated measurements are almost identical for all present four samples. This leads to the results that the calculated dose rates given by these samples were constant with time as shown in Fig. 49.4. The slight changes with time are shown for HPD#1328R04 and in HPD#1329R05. However, these are probably due to the geometric arrangement in measurement because corresponding similar changes of the intensity of 40K are observed in these samples.

Fig. 49.4
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The change in the dose rates from the retrieval of the samples up to 90 h. No change with time was observed

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The immediate decrease of radioactivity of the bulk sample, which is sometimes observed on the ship in a time scale of several hours, would possibly be due to the decay of short lived nuclei or loss of radon absorbed on the surface of the sample. However, the reason was not found in the present study.

The present study showed the ability of NaI detector, the resolution of which is much less than the pure Ge gamma ray spectrometer, to detect 228Ra in the samples. As 228Ra in the samples indicates that their ages are young, less than several 10 years, mapping of rough ages of sulfide deposits at the sea floor would be possible when such detection is done during the dives by submarine vehicles.