Use of 137Cs standard source for the calibration of 125I monitors

N INS I-U" ENT'S SMETMODS IN PHYSICS RESEARCH

Nuclear Instruments and Methods in Physics Research A312 (1992) 11-16 North-Holland

Section A

Use of "'Cs standard source for the calibration of

125 ,

monitors

Y. Kawada a and Y. Hino b " Faculty of Engineering, Seikei University,

Kichijoji-kita 3-3-1, Musashino, Tokyo 180, Japan ~' Electrotechnical Laboratory, Unrezono 1-1-4, Tvukuba, lbaraki 305, Japan

In order to confirm the suitability of the KX-rays from t ;QCs for calibration purposes, their pulse-height distributions were recorded under various conditions for a scintillation detector with a thin Nal(T0 crystal and a low energy photon spectrometer comprising a high-purity Ge detector. In the use of these detectors, the pulse-height spectrum due to 32 keV KX-rays is significant even in the presence of a dominant background of 662 keV -y-rays, and the full energy peak is well separated from this background. It is shown that the background continuum can be corrected by the absorber technique . The reliability of the absolute intensity of KX-rays is discussed, and our experimental results are presented . As an example of the applications, we describe how 125 1 monitors were calibrated by the use of "QCs standard source . 1. Introduction 137Cs is one of the most useful radionuclides for calibration purposes, and is used as a 662 kcV -y-ray intensity/ energy standard, a medium energy ß-ray intensity standard and an internal conversion electron reference standard . Its simple decay scheme and long half-life (30 yr) are advantageous features for routine calibrations. In addition, this nuclide is widely used in various fields of application, being very easy to obtain. Owing to these features and circumstances, standard sources of 137 Cs are in common use in most laboratories in the forms of an aqueous solution, a deposited point source or an extended large area source_ In the present work, we demonstrate the usetulness of this isotope as an X-ray (low energy photon) intensity standard. We discuss the reliability of the absolute intensity data for the K X-rays based upon previously reported values and our experiments . To demonstrate the validity of "'Cs standard sources as low energy photon intensity standards, 125 1 monitors were calibrated with their use .

2. Observed pulse-height spectrum As shown in fig . 1, about 95% of the ß-decays populate the 662 keV metastable state of " 7 Ba, whose half-life is 2.55 min. The transition from this state involves the internal conversion process, giving rise to 32 keV K a X-rays (5.5%) and 36 keV KR X-rays (1 .3%). Further details are given in the succeeding section . Note that the KX-rays are not coincident with any other radiations, which allows measurements free from the summing effect .

137 568 a Fig. l. Decay scheme of 1;7 Cs . 2.1 . Spectra observed with a Ge detector

Fig . 2 shows the observed spectra obtained with a high-purity coaxial Ge detector. The crystal is 35.7 mm in diameter and 13.3 mm in depth. The energy resolution is 270 eV FWHM at 5.9 keV and 540 eV FWHM at 122 keV. In spite of a fairly large active volume, the influence of the intense background of 662 keV -y-rays remains small . Peaks corresponding to K,, X-rays and K. X-rays are separated completely, and further separation of K,,, and K,,2 peaks is clearly seen, permitting a reliable quantitative analysis of these neighbouring peaks. In the energy region around these X-ray peaks, no other interfering peak was found. 2.2. Spectra observed with a Nal(TV scintillation detector

An example of an X-ray spectrum observed by the use of a 2 mm NATO scintillation detector is shown in fig . 3 as curve A. The detector diameter is 37.5 mm,

0168-9002/92/$05 .00 3 1992 - Elsevier Science Publishers B.V. All rights reserved

I(b). CALIBRATION STANDARDS

Y. Kawafta, Y. Hino /Use of 1-z7Cs for calibration

12

0 C C

a r U

11 Gl

C O U

1000

2000 Channel Number

3000

4000

Fig. 2. Pulse height spectra for "' Cs KX-rays obtained with the LO-AX detector . Source-detector distance 20 cm .

and its window is of 0.1 mm beryllium . In this measurement, the "7 Cs source was covered with 2 mm polyethylene (0.21 g/cm 2 ) in order to absorb ß-rays . The attenuation of K X-rays is calculated to be 4.3% . Even in the presence of a dominant background of 662 keV -y-rays, the Compton continuum lying under the peak

remains small provided that a thin crystal is used. This finding suggests the use of 137CS X-rays even for the calibration of Nal scintillation detectors. In order to separate the Compton background lying under the peak, the pulse-height spectrum was recorded with a I mm thick Cu absorber which was

2000

1600

O 43

C

ô

U

800',

400

200

400 Channel

600 Number

800

1000

Fig. 3. Pulse height spectra for "'Cs K X-rays obtained with a NATO scintillation detector of 2 mm thickness. The source was covered with 11 .21 g/cm 2 polyethylene absorber . A: without Cu absorber, B: with Cu absorber.

13

Y. Kawada, y. Hino / Use of 137Cs for calibration

placed in front of the window of the crystal, and the result is shown in fig. 3 as curve B. The spectrum obbained with the Cu absorber shows the Compton continuum lying under the X-ray peak . Especially in the higher energy region, the two curves overlap completely, which suggests that the difference of pulse rates with and without a Cu absorber gives the real response of K X-rays, although strictly one should allow for the 5% attenuation of 662 keV -y-rays in the 1 mm thick Cu absorber. To estimate the contribution to the background of scattered photons from surrounding materials, pulse-height distributions were also recorded with a 5 cm thick iron block and with a 5 cm thick lead block located closely behind the source . However, as shown in fig. 4, no distinct change was observed in the spectrum except in the energy region above 50 keV. These experiments support the view that the continuum component lying under the K X-ray peak was mainly due to Compton electrons produced in the scintillator, and that this contribution may be corrected by use of an absorber . The background continuum increasing with increasing thickness of the scintillator, but X-rays from a "'Cs source can be still used even in the case of a 1 cm thick crystal . 137CS 3. Absolute intensities of KX-rays in the decay of

To use a 137CS source as the intensity standard for low energy photons, the emission probabilities of the

K X-rays must be determined with an acceptable accuracy. In principle, the intensities, p,.. and PK~s, of Ku Xrays and K . X-rays per decay of "'Cs can be deduced in the following manner; PK« = Pl

and pK ,s-pl

aK

1 + al . wK l aK

1 +a _i-

WK (

/'7K,,  77K

1 +r1K, ./n K li

,

where a-,- and a K represent total and K internal conversion coefficients, (OK is the K fluorescence yield, 71K  and 77K11 are relative intensities of K,,, a td K,3 Xrays, and p , is the fractional ß-branch to the metastable state. Adopting a-r = 0.1097 ± 0.0007 00, a K = 0 .0890 ± 0 .0008, F I = 0.9443 ± 0.0001 as quoted from evaluated data in the Nuclear Data Sheets (NDS) [1], and (OK = 0 .901 ± 0.02, Tj K 13/ 17 K  = 0.235 ± 0.012 as given in the Table of Isotopes, Wh ed . [2], the following results were obtained : p K', = 0 .0131 ± 0 .0006 . pK = 0 .0553 ± 0.0017, The present calculations were compared with other reported data in table 1 . Of these data, the values given in NCRP-58 [4] and the experimental values of Debertin and Pessara [5] are in good agreement with the present calculation. However, the evaluated data

c c m

L U

N C O

'rig. 4. Pulse height spectra for "QCs source obtained with a NaI(T!) scintillation detector of 2 mm thickness . A: without any scatterer, B: with a 5 cm iron block behind the source . !(h) . CALIBRATION STANDARDS

Y. Kawada, Y. Htrto / Use of '-;7

14

Table 1 Emission probabilities of KX-rays from 1;7Cs Emission probability Method Author/ reference evaluation K,, = 0.0567 ± 0.0019 NCRP-58 [4] Kp = 0.0134 ± 0.0005 Table of Isotopes 7th ed. [3]

evaluation

K,, = 0.063 ±0.002 Kw = 0.012 ±0.001

K [I 2 = 0.0029 ± 0.0003

K = 0 .0560±0.0012 Debertin & Pessara [5] calibrated spectrometer K,3 =0.0131±0.0006

Present work

calculation

Present work

calibrated K,, = 0.0560 ± 0.0008 spectrometer Kw = 0.0105 ± 0.0004

K --- 0.0553 ±0.0017 K11 = 0.0131 ± 0.0006

KP2 = 0.0030 ± 0.0002

tabulated in appendix (3) of The Table of Isotopes, 7th ed. [2], are not consistent with other results, and out of range of the quoted uncertainties. The absolute intensities of Kn and K ß X-rays can be deduced from the decay data and atomic data. However, the associated uncertainties are large, mostly due to the large uncertainties of coK and '7K,/71K,. . As a result, the reliability of experimental values is likely to exceed that of the calculations. Debertin and Pessara [5] determined the absolute intensities of K a and KR X-rays by the use of a calibrated low energy photon spectrometer, and obtained results consistent with the values given in NCRP No. 58. In order to check the reliability of the absolute data, we also measured the emission probabilities of K,, and KR X-rays per decay of 137Cs with a high-purity germanium spectrometer (EG & G Ortec LO-AX reverse-electrode closed end coaxial type) of the Electrotechnical Laboratory (ETL) . Significant crystal sensitivity (35.7 mm in diameter and 13.3 mm in depth), a thin inactive layer (less than 0.3 gm) and a beryllium window (0.5 mm thick) ensured a nearly flat response in the energy range from a few keV to 100 keV . The energy resolution was 270 eV at 5.9 keV and 540 eV at 122 keV. Calibrations had been made using standard sources of 11 ' In. 139Ce, 241Am, 13313a and 57 Co, and the calibration curve at a source-detector distance of 20 cm is shown in fig . 5. It should be noted that the response is completely flat in the range below 60 keV after correction for the escape effect . Detailed procedures are given in ref. [6]. The constancy of the calibrations was frequently checked by the use of an 241Am source . Measurements were carried out for three deposited sources of 137CS prepared from a standard solution whose radioactivity concentration had been determined by efficiency tracing used 134 Cs as the tracer. The uncertainty was estimated to be 0.3% (1o- ). An

for calirtCs 10 3

1C" 1412-

"d

I .0

w 08

-

OG 04 02 000

50

100 Pholon

150 Energy (keV )

200

250

Fig. 5. Efficiency calibrations of LO-AX photon spectrometer corrected for absorptions in extraneous materials between source and detector crystal. The dotted line is for the full energy peak, while the solid line includes the full energy peak and the escape peak [6]. example of the observed spectrum has already been shown in fig. 2. The results of the above measurements are pK. = 0.0560 ± 0 .0008, p KOi

= 0.0105 ± 0.0004,

pKw,

= 0.0030 ± 0.0002,

= PK O I +PKaz = 0 .0135 ± 0.0005 . These results are in good agreement with those obtained by Debertin and Pessara [5] within the stated uncertainties .

PK, j

4. Calibration of 125 1 monitors by the use of a 137CS standard source To demonstrate the usefulness of 137Cs sources as low energy photon intensity standards, we tested the possibility of the calibration of 125 1 monitors with their use . 1251 is an important and widely used radionuclide especially in the field of nuclear medicine . Since this nuclide emits only 28 keV and 35 keV photons (X/Y), special instruments designed for effective detection of low energy photons are sometimes used for the measureIIne nts, e.g. for contamination surveys and monitoring. These use mostly a scintillation counter with a thin NATO crystal . Routine calibration of such instruments with 1251 sources is not convenient, since the half-life is short. As shown in fig . 6, the observed pulse-height spectra for 1251 and 137 Cs K X-rays are similar to each other, and the energy difference is only 4 keV. However, use of a plastic absorber is essential to eliminate the effect of ß-rays . Considering these aspects and

Y. Kawada, Y. Flino / Use of 1-170 for calibration

C C

m

t U

C) C O U

Fig. 6. Pulse height spectra for 137Cs and 1251 obtained with a Nal(TI) scintillation detector of 2 mm thickness. The continuum lying under the peaks was obtained with a 1370 source covered with Cu absorber.

expecting a nearly flat response of such instruments in this energy region, practical calibration in terms of radioactivity is considered to be possible with an acceptable reliability . If a po-sible summing effect is neglected in the measurements of 125 1 and the window setting of the single channel pulse height selector of the measuring

instruments covers the peaks of both 1251 and 1370 KX-rays, the calibration factor K for 125 1 measurements is given as K=

n137( (

PK +PK,)

1 /f1)(R137

1

- Ri37/f2) P125

,

mC

C

m

t U

4-3

C

O U

200

400

600

800

1000

Channel Number 137Cs obtained with a Nal(TI) scintillation detector of 2 mm thickness. for 1 and Fig. 7. Pulse height spectra 129 IN . CALIBRATION STANDARDS

16

Y. Kawadu, Y. Hirtt) / Usc, t)f

where R137 and R1 .37 represent the responses (count rates or meter readings) of the measuring instruments under test to the 137CS standard source with and without the Cu absorber (radioactivity : 11,37), f', is the attenuation factor of K X-rays in the plastic absorber for ß-rays and ,/', is the attenuation factor of 662 keV -y-rays in the Cu absorber. p12S denotes the sum of emission probabilities of the 35 keV l'-rays and Te K X-rays per decay of "1 . The activity, n 12, of the 1251 source will be given as 11 125 = KR ,25 ,

where R î25 is the response of the instrument to the 1251 source . However, if the source is located near the detector in the measurements of 12-11, the observed pulse rate may decrease due to the simultaneous detection of two photons (summing effect between X-rays arising from K capture and X-rays which originated from the internal conversion process or 35 keV -y-rays). This effect can be corrected by a simple calculation as a function of the apparent efficiency obtained with the ';7Cs source . In our laboratory, the calibration factor of a commercially available i25I monitor with a 2 mm Nal scintillator was compared with that obtained using a 1251 standard, and agreement within 5% was attained for several cases. In some cases 12`'1 is used as an alternative standard to 1251 . The observed spectrum of 1291 is also shown in fig . 7. The spectrum is very similar to those of 1251 and 137Cs. This indicates that a 137Cs standard source can be also used as an alternative intensity standard to 129 1 .

1,s7(,

Jùr calibration

. Conclusion The usefulness of K X-rays from a 137Cs standard source has been discussed and demonstrated with reference to two types of detector . Even in the presence of a dominant background of 662 keV -y-rays, the peak(s) for K X-rays are well separated from the background continuum . The absorber technique has been shown to be useful for determining the background continuum in the case of the calibration of a Nal(TI) scintillation detector. For the use of 137CS sources as low energy photon standards, the absolute intensities of the K X-rays must be known . Up to the present time, uncertainties of the order of a few percent are still found in the reported data. Further improvement of the absolute intensity data will give a more important role to the "7Cs source . References [1] L.K. Perker, Nucl. Data Sheets 59 (1980) 767. [2] C.M. Lederer and V .S. Shirley (eds.), Table of Isotopes, 7th ed. (Wiley-Interscierice, 1978) Appendices p . I1 . [3] Ibid., Appendices p. 4. [4] National Council on Radiation Protection and Measurements, NCRP Report No. 58, 2nd ed. (1985) . [51 K. Debertin and W. Pessara, Int . J. Apl . Radiat . lsot. 34 (1983) 51 .5. [6] Y. Kawada, Y. Hino and W. Gatot, Nucl. Instr. and Meth. A286 (1990) 539.