Applied Radiation and Isotopes 81 (2013) 261–267
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Absorption and backscatter of internal conversion electrons in the measurements of surface contamination of 137Cs A. Yunoki a,n, Y. Kawada a, T. Yamada a,b, Y. Unno a, Y. Sato a, Y. Hino a a National Metrology Institute of Japan, National Institute of Advanced Industrial Sciences and Technology, Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan b Japan Radioisotope Association, Hon-Komagome, Bunkyo, Tokyo 113-3941, Japan
H I G H L I G H T S
Counting efficiencies for internal conversion electrons from 137Cs were measured, and compared with those for β-rays. Electron-X coincidence technique was employed. A thin NaI(Tl) scintillation detector was used for X-ray detection. Backscattering fractions of electrons and beta particles were studied by similar experiments.
art ic l e i nf o
a b s t r a c t
Available online 29 March 2013
We measured 4π and 2π counting efficiencies for internal conversion electrons (ICEs), gross β-particles and also β-rays alone with various source conditions regarding absorber and backing foil thickness using e-X coincidence technique. Dominant differences regarding the penetration, attenuation and backscattering properties among ICEs and β-rays were revealed. Although the abundance of internal conversion electrons of 137 Cs-137Ba is only 9.35%, 60% of gross counts may be attributed to ICEs in worse source conditions. This information will be useful for radionuclide metrology and for surface contamination monitoring. & 2013 Elsevier Ltd. All rights reserved.
Keywords: 137 Cs Internal conversion electrons e-X coincidence technique Counting efficiency Backscatter
1. Introduction As a result of the Fukushima Nuclear Power Plant accidents, part of east Japan was contaminated with radioactive materials. Various kinds of radionuclides of short half-lives such as 131I rapidly decayed and only 134Cs and 137Cs remained as major sources of contamination in the present stage. Among these nuclides, 137Cs emits a considerable proportion of internal conversion electrons (ICEs) via 137mBa metastable state (T1/2 ¼ 2.55 min) as shown in Fig. 1. While the mean energies of continuous β-rays are about 174 keV (94.6%) and 415 keV (5.4%), the ICEs have line spectrum with weighted mean energy around 630 keV (624 keV: 7.62%, 656 keV: 1.42% and 660 keV: 0.33%) as quoted in Bé et al. (2006). These ICEs are not coincident with any β-particles nor 662 keV photons, but are in coincidence with associated Ba K X-rays (XKα: 32 keV, XKβ1: 36 keV, and Xkβ3: 37 keV). This dominant difference in the features between ICEs and continuous β-emissions will give difference in counting efficiencies (Korun and Modec, 2011), and hence the effects of ICEs should always be in mind, especially when the source
n
Corresponding author. Tel.: +81 29 861 3470; fax: +81 29 861 5673. E-mail address:
[email protected] (A. Yunoki).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.03.014
geometry is unfavorable such as in the cases of surface contamination measurements of the environmental samples. In order to make clear these situations, we constructed a special 4πβ-X coincidence counting system which consists of a thin NaI(Tl) detector and a rectangular box-shaped 4π counter with a thin wall, and measured the counting efficiencies of ICEs from 137Cs sources with various source conditions using electron-X coincidence technique. The total β-particle emission rate including ICEs is determined from the count rate in the 4πβ-channel. Combining these two data, the count-rate of β-rays excluding the effect of ICEs can be evaluated with an acceptable accuracy. With this counting configuration, the difference of backscattering of ICEs and continuous β-rays was also derived. These data will provide useful information in measurements and evaluations of 137 Cs radioactivity by means of β-ray detection. 2. Experimental 2.1. Experimental arrangement Fig. 2 shows the diagram of the experimental arrangement. The detector part consists of a 3.75 cm diameter 2 mm thick NaI(Tl) scintillation detector and a rectangular shaped 4π-β gas flow counter. Argon plus 10% methane gas (P-10 gas) is used as flowing gas of 4π detector, under 1500 V operating high voltage. Both top
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and bottom walls are made of 0.1 mm aluminum foils to keep efficient penetration of 32 keV X-rays and also to minimize backscattering. In order to prevent penetration of β-particles, a 6 mm thick polymethylmethacrylate (PMMA, with 1.2 specific density) circular disc is placed in front of the NaI(Tl) scintillation detector. This is thick enough compared with the range of β-particles with Emax ¼1176 keV that is about 400 mg cm−2, whereas the attenuation of 32 keV X-rays in this PMMA absorber is about 11%. In the β-channel, the signals from top-β and bottom-β detectors were amplified, discriminated and counted separately, and the total 4π counts were obtained with another counter after passing an OR circuit connected between the two β-channels. As electronics modules, conventional commercially available ones were used except for the delay and pulse shaper circuits that were laboratory made. Seiko EG&G Model 7700 MCA was employed for pulse height analysis. The
Fig. 1. Decay scheme of
137
Cs.
dead time was set at 10 μs for both β- and γ-channels. The resolving time width is 0.6 μs for the γ-channel and 2 μs for the β-channel. The sources were prepared by dropping quantitative amount of standardized 137Cs solution on a VYNS film (25 μg cm−2) coated with gold (20 μg cm−2) to render it electrically conductive. In the course of source preparation, 1:3000 diluted Ludox-SM 30 solution was added as a detergent. The activity of each source was about 5 kBq. 2.2. 4π electrons-X coincidence counting As was mentioned in previous papers (Kawada and Hino, 1992; Kawada et al., 1996), by the use of thin NaI(Tl) scintillation detector, we can achieve efficient and selective detection of Ba K-X-rays from 137Cs source even under existence of intense 662 keV γ-rays. Spectrum A of Fig. 3 shows an example of Ba Kα X-rays (E 32 keV) observed with our system. The slight bump observed at the right side of the peak might be caused by Ba Kβ1 and Kβ3 X-rays (E 36 keV). The underlying continuum is mainly caused by Compton scattering of 662 keV photons, which are not in coincidence with any β-particle nor ICEs. Thus, the peak area after subtracting the underlying continuum is due to Ba K-X-rays (32 keV to 37 keV). The ratio of the coincidence counts to the peak area after subtracting the underlying continuum gives the counting efficiency of β-counter for ICEs alone. It was already demonstrated that the counting efficiency of 4πβcounter for internal conversion electrons remains very nearly equal to unity even for several tens of keV, if the sources are prepared on thin VYNS films and adequately treated with use of a seeding agent (Kawada, 1972). Accordingly, it is likely that counting efficiency for 630 keV ICEs is very nearly equal to unity for sources prepared on a VYNS film. In such a case, the underlying continuum should be diminished mostly provided that the X-ray spectrum is recorded under coincidence gating mode with the 4πβ-signals. The coincidence gating spectrum is shown in Fig. 3 as curve B. Indeed, the continuum part is significantly diminished and only the real peak remains unchanged. Very small residual in the continuum part might be mostly due to Bremsstrahlung as will be discussed later. On the other hand, if the MCA is operated with anti-coincidence mode, gated with β-channel signals, the peak area is diminished and the continuum part remains unchanged as shown in curve C of Fig. 3. Trace bump in the peak area might be accidental ones caused
Fig. 2. Diagram of experimental arrangement.
A. Yunoki et al. / Applied Radiation and Isotopes 81 (2013) 261–267
263
6 4
Counts/3000s
2
A:Normal
100 6 4 2
C:Anti-Coincidence Gating
10
B:Coincidence Gating
6 4 2
1 0
200
400
600
800
1000
Channel Number Fig. 3. Pulse height spectra obtained with 137Cs source ( 5 kBq) using 2 mm thick NaI(Tl) detector in the present 4πe-X coincidence arrangement. The dominant peak is due to Ba K-X-rays. (A) Normal spectrum without gating, (B) Spectrum gated with signals from 4π counter, (C) Spectrum anti-gated with signals from 4π counter.
by counting losses due to dead time in the β-channel. From curve C, we can study the shape of the base line. In the determination of counting efficiency for ICEs, accidental coincidences were corrected by the similar manner described by Campion (1959). As was already mentioned, trace coincidence component suspended in the underlying continuum was also subtracted from coincidence counts. The reason of producing such coincidence component might be Bremsstrahlung originating from the side wall of the 4π-counter. The counting efficiency thus determined includes the effects of counting losses due to dead time of β-channel, which were corrected in the final stage. First measurement was done for a source prepared by a conventional manner on VYNS film. In order to balance the counting rates in the top-half and in bottom-half, both surfaces of the source were covered with similar VYNS films. The determined counting efficiency was εIC ¼0.995 70.006 (1s) for no additional aluminum absorber. Count rate from the 4πβ-counter gives emission rate of gross β-particles (sum of β-particles and ICEs) from the source. From the counting efficiency for ICEs and total β-particles emission rate including ICEs, we can derive the emission rate only for β-rays with an acceptable accuracy provided that self-absorption and foil-absorption in the thin VYNS film can be evaluated from the source activity of the standard solution dropped. Our typical result of sum of the self-absorption and foil-absorption was 3%. Adopting similar technique successively with increasing the thickness of laminated aluminum sandwiching absorbers, we can determine the counting efficiencies of ICEs and β-rays separately for a 137Cs sources with absorbers of various thickness. Fig. 4 represents the counting efficiencies of 4πβ-detector for ICEs and for gross β-particles in the forms of linear (right) and logarithmic (left) scales. From the counting efficiency, εIC, of 4πβ-detector for ICEs and the gross β count rate, nGross, the counting rate, nβ, only for β-particles can be calculated according to: p ðα ε þ εβγ Þ ; nβ ¼ nGross −n0 1 T IC 1 þ αT
ð1Þ
where p1 denote the fraction (0.9436) of β-rays which populates to the 137mBa metastable state, αT is total internal conversion coefficient (0.110) and n0 is the source activity which is determined from the activity concentration of the standard solution and the sourcemass dropped in the source preparation. Otherwise n0 can be estimated with practically acceptable accuracy with a simple 4πβcounting after correcting the self- and foil-absorptions. Efficiency of the 4πβ-detector for γ-rays denotes as εβγ, which is small enough (less than 0.5%) and usually can be neglected. The counting
efficiency εβ only for β-rays (excluding ICEs) is reduced as: ηβ : εβ ¼ η0
ð2Þ
All these calculations were performed after correcting the counting losses due to the dead time. The counting efficiency of the 4πβ-detector only for β-rays thus calculated is also shown in Fig. 4 as a function of the absorber thickness. It should be noted that the efficiency for ICEs remains nearly unity over a considerable range and then gradually decreases with increasing absorber thickness. The relative standard uncertainty of the efficiency is estimated to be 2% at the maximum from the counting statistics, subtraction of base continuum, etc. On the other hand, efficiencies for gross β-particles and for β-particles decrease rapidly with increasing absorber thickness. The lines in the figure are fitting traces according to the least-squares method. Best fittings were obtained using Lorentzian function: n o f ðxÞ ¼ K 0 þ K 1 = ðx−K 2 Þ2 þ K 3 ; ð3Þ for ICEs and double exponentials: f(x) ¼K0+K1exp(−K2x)+K3exp(−K4x),
(4)
for gross β-particles and for β-ray only. x denotes the aluminum absorber thickness in terms of mg cm−2. In all cases, the fittings are quite satisfactory, and the values of χ2 are very small. Remarkable differences of absorption data between ICEs and β-rays have revealed the importance of the counting contribution of ICEs especially in the worse source conditions such as measurements of surface contamination of environmental samples. As can be seen in Fig. 4, the contribution of ICEs to the gross counts amounts to 20% or more for sources covered with thick absorbers. 2.3. 2π electrons-X coincidence counting Nearly the same experiments as described above were carried out using the top-half of the 4πβ-detector as a 2π electron(β)-X detector configuration. 2.3.1. Measurements of the backscattering coefficient In order to clarify the difference of backscattering for ICEs and βrays, we performed similar experiments successively with backing aluminum foils of various thicknesses which were contacted below a bare source on VYNS film. To estimate possible contribution of the backscatter effects from counter wall and/or counting gas in bottom-βcounter, coincidence counting between top-β and bottom-β pulses was
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Counting Efficiency(%)
100 80 Internal Conversion Electrons
60 40 Gross
20
β−Rays only 0
20
40
60
80
100
120
Aluminum Absorber Thickness (mg/cm2)
Counting Efficiency(%)
100 6 5 4
Internal Conversion Electrons
3 2
10
Gross
6 5 4
β−Rays only
3 2
0
20
40
60
80
Aluminum Absorber Thickness
100
120
(mg/cm2)
Fig. 4. 4π counting efficiencies for ICEs, gross β-particles and β-rays emitted from 137Cs sources with absorbers. The data are plotted both in linear and logarithmic scales. The lines are trace of least-squares fitting curves using Lorentzian (for ICEs) and double exponential function (for gross β-particles and β-rays). (a) 4π Counting (Linear Scale) and (b) 4π Counting (Log Scale).
recorded, and this contribution was subtracted from the top-β counting. The counting efficiencies for ICEs and gross β-counting were determined by the use of the same manner as described in subSection 2.2, and the backscatter fractions, ηIC and ηGross, for ICEs and gross β-particles, respectively, as: ηIC ¼
εIC εgross −1; ηgross ¼ −1 εIC0 εgross;0
ð5Þ
where εIC0 and εgross,0 are efficiencies for ICEs and gross β-particles, respectively, without backing aluminum foil. In the first measurement without backing, the source was covered with another VYNS film coated with gold in order to compensate the effects of backscattering from the base VYNS film. The results are shown in Fig. 5 for both cases as a function of thickness of backing aluminum foil. In the calculation, the maximum deviation of 0.03 may arise to the backscatter fraction. As seen in the figure, the backscattering component grows rapidly with increasing thickness of the back foil in the case of gross β-particles as compared with that of ICEs. The saturated value of the backscattering coefficient for ICEs is higher than the gross β-rays one. These results are very consistent with the results on backscattering characteristics appeared in literature, such as Price (1964), in which the backscattering of β-rays from 210Bi (Emax ¼ 1.17 MeV) were compared with those from 131I (Emax ¼0.6 MeV).
2.3.2. Counting efficiencies for ICEs and β-rays in 2π geometry In cases of 2π geometry where backing material thick enough is placed below the source, the absorbing curves may differ from those of the 4π geometry, owing to the existence of backscattered components. In order to clarify such a situation, 2π counting efficiencies of ICEs and gross β-particles were measured using a source with a 1 mm thick (270 mg cm−2) aluminum backing. The measurements were done successively with adding the aluminum absorber above the source. The results are summarized in Fig. 6 (linear and logarithmic scales) as a function of the aluminum absorber thickness on the source. In this figure, calculated results of the efficiency only for β-rays excluding ICEs are also shown. Whereas the counting efficiency for 4π counting remains nearly equals to unity for a considerably long range, appreciable dropping of the efficiency is observed even for ICEs in case of 2π counting. This is because the energy of scattered ICEs decreases considerably and the spectrum broadens. The lines in Fig. 6 are trace of the best fitting by means of the least-squares algorithm. The form of all fitting functions is double exponential. In the case of ICEs, the fittings were made dividing the fitting region into two parts: xo 10 mg cm−2, and x 410 mg cm−2. In all cases, the fittings are excellent.
A. Yunoki et al. / Applied Radiation and Isotopes 81 (2013) 261–267
ICEs
0.3
Backscatter Fraction
265
Gross β (Including ICEs)
0.2
0.1
0.0 0
50
100
150
200
Thickness of Aluminum Scatterer
250
(mg/cm2)
Fig. 5. Backscatter fraction as a function of thickness of aluminum backing. Closed circles are for ICEs and open ones for gross β-particles. Fittings were made according to the double exponential function.
Counting Efficiency (%)
100 80 60
Internal Conversion Electrons
40 20
Gross β−Rays only
0 0
20
40
60
80
Aluminum Absorber Thickness
100
120
(mg/cm2)
100
Counting Efficiency (%)
8 6 4 2
Internal Conversion Electrons
10
8 6
Gross
4
β−Rays only
2
1 0
20
40
60
80
Aluminum Absorber Thickness
100
120
(mg/cm2)
Fig. 6. 2π counting efficiencies for ICEs, gross β-particles and β-rays emitted from 137Cs sources with absorbers. The data are plotted both in linear and logarithmic scales. The lines are trace of least squares fitting curves using double exponential function. See also text. (a) 2π Counting, Linear Scale and (b) 2π Counting, Log Scale.
2.3.3. Counting efficiencies for ICEs and β-rays emitted from uniformly distributed sources From the 2π counting efficiency data for various absorber thicknesses obtained with a thin source on thick backing foil, we can calculate the emission probability of ICEs, gross β-particles
and β-rays, from a thick source whose radioactivity is distributed uniformly. As was already mentioned, the best fitting functions for responses of 2π counting versus the thickness absorber, x, are all expressed in the form of double exponential function.
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A. Yunoki et al. / Applied Radiation and Isotopes 81 (2013) 261–267
Table 1 Best coefficients of fitting function in the least-squares fitting for absorption curves. Curve name
Fitting function
K0
K1
K2
K3
ICEs for 4π
n o f ðxÞ ¼ K 0 þ K 1 = ðx−K 2 Þ2 þ K 3
-0.1573
11295
7.515
9806
Gross for 4π β-rays only for 4π ICEs for 2π
f ðxÞ ¼ K 0 þ K 1 expð−K 2 xÞ þ K 3 expð−K 4 xÞ Same as above Same as above
Gross for 2π β-rays only for 2π
Same as above Same as above
0.037 0.0122 0.440 -0.299 0.0240 0.0078
0.710 0.772 0.126 0.471 0.392 0.418
0.0335 0.0430 0.0452 0.00571 0.0389 0.0471
0.215 0.183 0.129 0.487 0.222 0.227
100
Counting Efficiency (%)
8 6
K4
0.280 0.405 0.00442 0.00563 0.370 0.424
χ2
Remarks
0.0014
Lorentzian
0.0009 0.0003 0.0002 0.0006 0.0003 0.0003
x ≤10 mg=cm2 x 4 10 mg=cm2
B: ICEs (Thick source)
4
A: ICEs (Thin source with absorber) D: Gross (Thick source)
2
10
8 6
F: β− Rays (Thick source)
4
2π Geometry (Backing 1mm Al)
2
1
C: Gross (Thin source with absorber)
E: β−Rays (Thin source with absorber)
8 6
0
20
40
60
80
Absorber/Source Thickness
100
120
(mg/cm2)
Contribution of ICEs to Gross Count
Fig. 7. Comparison of 2π counting efficiencies (escape probabilities) from ICEs, gross β-particles and β-rays from thin source covered with aluminum absorber and those from thick homogeneous source. (A) ICEs from thin source with absorber, (B) ICEs from thick homogeneous source, (C) gross β-particles from thin source with absorber, (D) gross β-particles from thick homogeneous source, (E) β-rays from thin source with absorber, (F) β-rays from thick homogeneous source.
1.0 0.8
2π (Thin souce with absorber) 0.6
4π (Thin source with absorber)
0.4 0.2
2π (Thick homogeneos source)
0.0 0
20
40
60
80
100
120
Absorber/Source Thickness (mg/cm2) Fig. 8. Contribution of ICEs to gross β-particles for 4π and 2π geometry.
The coefficients obtained for the best fitting according to the least squares criteria are shown in Table 1 for every three cases together with those for 4π counting. Using these fitting functions, 2π escape probabilities, F(s), of ICEs, gross β-particles and β-particles from a thick 137Cs source on thick backing material can be calculated by the following way: FðsÞ ¼
1 s
Z 0
s
K3 K1 1−expð−K 4 sÞ 1−expð−K 2 sÞ þ f ðxÞdx ¼ K 0 þ K2s K4s
ð6Þ
where s represents the source thickness in terms of mg cm−2. In the case of ICEs the integration was made in two parts, and
combined by: FðsÞ ¼ F 1 ðsÞ F 0 ðsÞ ¼
f or s ra
a s−a F 1 ðaÞ þ F 2 ðsÞ s s
f or s 4a
ð7Þ
The calculation results of individual 2π-escape probabilities for ICEs, gross β-particle and β-rays for a thick source are shown in Fig. 7 together with those for a thin source covered with absorber for comparisons. The attenuations are reduced by a factor of about two in the case of homogeneously distributed source as compared with those for thin source covered with absorber.
A. Yunoki et al. / Applied Radiation and Isotopes 81 (2013) 261–267
3. Concluding remarks By the use of electron-X coincidence counting using a thin NaI (Tl) scintillation detector and a thin wall 4π detector, a novel technique has been developed for determining individual counting efficiencies for 630 keV conversion electrons, gross β-particles, and β-rays only from 137Cs source with various conditions. This technique was applied for 4π and 2π counting efficiencies for ICEs, gross β-particles and also β-rays alone with various source conditions about absorber and backing foil thickness. Using the fitting function of these data, emission probabilities of ICEs, gross β-particles and β-rays from thick homogeneous source were derived, which revealed dominant differences regarding the penetration, attenuation and backscattering properties among IECs and β-rays. Although the abundance of internal conversion electrons of 137 Cs-137Ba is only 9.35%, 60% of gross counts may be due to ICEs in worse source conditions. In Fig. 8, the contribution ratios of ICEs to the total counting are summarized for both 4π and 2π counting. The differences found in this study are consistent with change of the β-ray spectra observed after penetrating aluminum absorbers of various thicknesses (Kawada et al., 2008). These data and knowledge obtained in this study will be useful for various aspects in the fields of radionuclide metrology and in surveillance of
267
radioactivity contamination based upon β-ray detection. This study was carried out using aluminum absorbers, but the data obtained here can be applied for cases of other materials, if absorber or source thickness is expressed in terms of mg cm−2, and atomic number of the material are not seriously different.
References Bé, M., et al., 2006. Table of Radionuclides (Vol. 3—A ¼ 3 to 244) Monographie BIPM5 (2006) 98. Campion, P.J., 1959. The standardization of radioisotopes by the beta–gamma coincidence method using high efficiency detectors. Int. J. Appl. Radiat. Isot. 4, 232–248. Kawada, Y., 1972. Self-and foil absorptions of low energy internal conversion electrons in 4πβ-counting. Nucl. Instrum. Methods 98, 21–27. Kawada, Y., Hino, Y., 1992. Use of 137Cs standard source for the calibration of 125I monitors. Nucl. Instrum. Methods Phys. Res. A312, 11–16. Kawada, Y., Mogi, J., Hino, Y., 1996. Reduction of γ-ray background in the use of 32 keV X-rays from 137Cs sources by a gating method with a thin plastic scintillation detector. Nucl. Instrum. Methods Phys. Res. A369, 671–675. Kawada, Y., et al., 2008. Observation of β-ray spectra after penetrating absorbing materials. Appl. Radiat. Isot. 66, 819–822. Korun, M., Modec, P.M., 2011. Coincidence summing between X-rays and conversion electrons in 137Cs. Appl. Radiat. Isot. 69, 1263–1266. Price, W.J., 1964. Nuclear Radiation Detection, second ed. Mc-Graw Hill.