Nuclear Instruments and Methods in Physics Research A 383 ( 1996) 441-446 NUCLEAN

ELSEWIER

A calibration technique for gas-flow ionization half-lived rare gases

M. Yoshida”‘“, T. Oishi”, T. Hondab, T.

INS-NTS & METNODS IN PNYSICS

R%%T”

chambers with short

Torii’

“Japan Atomic Energy Research Institute, Tokai 319-I I, Japan ‘Institute of Radiation Measurement, Tokai 3 19. I I, Japan

‘Power Reactor and Nuclear Fuel De~&ppmer~t Corp.. T.mruga 919-12. Japan

Received 20 May 1996: revised form received 12 August 1996

Abstract

A calibration technique for gas-flow ionization chambers was studied for implementation of reliable radioactive gas

monitoring. Three radioactive gases with short half-lives of “3Xe, ‘“5Xe and “Ar were prepared by activating stable

isotopes and used for the calibration. On the basis of activity determination by the DLPC method, a gas-flow ionization

chamber used as a secondary standard was precisely calibrated in terms of ionization efficiency for each radionuclide. The influence of impurities in the 133Xe gas on calibration of gas monitoring instruments is also discussed. This technique is

considered to make the easy and reliable calibration of gas monitoring instruments possible.

1. Introduction

A calibration of gas monitoring instrument (hereafter called gas monitor) with radioactive gases is one of the most important procedures for increasing the reliability of

the measurements for radiation protection. Among the

various radioactive gases applied for the calibration pur-

pose, radioactive rare gases with relatively short half-lives have the following merits: 1) increase in the number of

useful energies of beta- or gamma-rays; 2) less contamina- tion remaining in calibrated gas monitors; 3) reduction of

the radioactive gas waste used for calibration. On the other hand, they are difficult to store and need to be simul- taneously produced for the calibration. However, since radioactive rare gases of 4’Ar, “‘Xe and lZSXe are easily

produced by (n,y) reaction, their use for the calibration is convenient for some facilities where thermal neutron fields

by nuclear reactors or accelerators can be utilized. On the calibration with the radioactive rare gases with

short half-lives, their activity concentrations in a gas monitor need to be known at the moment of calibration.

The activity can be usually determined by a direct method with internal gas counters [l-3] or by an indirect method with a calibrated radiation measuring instrument as, for

example, a 47ry ionization chamber [4]. In general. the direct method can provide more reliable standards in comparison with the indirect one. Gas monitors cannot be calibrated by directly connecting to the internal gas counter

* Corresponding author.

giving activity standards, because the air as their carrier

gas is an electronegative gas. Hence, such a gas-flow ionization chamber as can measure the activities in the air

is useful as a secondary standard instrument for the calibration of gas monitors. Then it is important to

evaluate beforehand the precise response of the ionization chamber on the basis of the direct method.

This paper describes the preparation of “‘Ar, ‘“‘Xe and

Iq5Xe for the purpose of gas-monitor calibration, and the

determination of their activities by the DLPC (diffusion in long proportional counter) method [2,3] which is one type

of the internal gas counting method. It also describes a new calibration method of a gas-flow ionization chamber used as the secondary standard on the basis of the activity

determination. In addition, the influence of the impurities contained in produced “3Xe gases on the calibration of

gas monitors is discussed.

2. Experiments

2.1. Preparation of radioactive rare gases with short

half-lives

The radioactive rare gases of “Ar, “‘Xe and “‘Xe can be produced in a thermal neutron field by activation of their respective stable isotope gases of 40Ar, j3’Xe and ‘34Xe. These radionuclides are valuable for the calibration

of gas monitors because of their appropriate beta- and gamma-ray energies and branch ratios [5]. The neutron

0168-9002/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved PII SO1 68-9002(96)00834-O

442 M. Yoshida et al. I Nucl. Instr. and Meth. in Phys. Res. A 383 (1996) 44-446

Table 1 Irradiation conditions for prepared rare gases

Objective nuclide Target gas enrichment [%] Neutron jhu qf~ Irradiation time [min] Cooling time Composition [%I [cm-‘s-‘1

“Ar

’ “Xe

’ “Xe

“‘Xe

‘ISXe

“‘AK > 99.9

“JXe: >99.6

’ “Xe: >99.9

13’Xe: >99.9

“‘Xe: <25

‘jZXe: 64.6

‘lJXe: ~25

6.0x10” 1 ~;~;:m~‘. 1.3x 10”

F_y 6.0X 10” 10

Fa*l 6.0x 10”’

&Therms,: 6.0x 10” 10

d,,\, : 6.0X 10”

6 hLr,n.,: 6.0X 10” 10

k.,: 6.0~ 10”’

4 Therma,: 6.0X 10” 10

k\, 1 6.0x 10”’

4h

4h

1.3 d

“‘Xe”: CO.6

14 d

“‘Xe”: CO.4

21 d

I1’Xem: 15.2 ““Xe”: <0.4

4’Ar: >99.9

“‘Xe: >99.9

“‘Xe: 85.0

‘llXem: 14.4

’ “Xe: 98.3

’ “Xem: 1.3

’ “Xe: 84.2

I1’Xem: CO.3

fields of nuclear research reactors in the Japan Atomic

Energy Research Institute were utilized for their pro-

duction. The 10 cm3 stable isotope gases contained in 20 cm’ quartz ampoules were irradiated for several minutes and converted into respective radioactive ones with an

activity of several MBq. The stable gases of 13’Xe and ‘24Xe were purchased from EURISO-TOP in France.

The concentration of impurity was estimated with a

HPGe detector. The radioactive gases packed in a small

glass tube (inner diameter: 8 mm, length:20 mm) was

placed 15 cm above the top surface of detector. The

counting efficiency of detector vs. gamma-ray energy was determined with the mixed-source and lJ3Ba standard

solutions supplied by LMRI in France, with which the same glass tube was filled as used for the gas measure-

ment. Measurements of the impurity concentration were made

in parallel during calibration works of gas monitors. Table

1 gives the enrichment of target gas, neutron flux, irradia- tion time, and the activity composition of some prepared

gases.

2.2. Measuring apparatus

The experimental system is illustrated in Fig. 1. The long proportional counter has a cylindrical cathode 40 mm

in inner diameter and 900 mm long. It also has a central

port where a radioactive gas can be injected through a

rubber packing to determine its activity. The counting gas of PlO (90% Ar and 10% methane) was used at a pressure

of 98.1 kPa. The signals from a pre-amplifier were counted after a linear amplifier, a discriminator and a dead-time

setter. The output pulses from the linear amplifier were simultaneously monitored with a pulse height analyzer to evaluate the count loss due to the discrimination level.

A 1500 cm3 cylindrical gas-flow ionization chamber

Gas injection port

ti:

Circulation pump

ICountmg gas inlet & exhaust

IGasonitors 1

Fig. 1. Measuring apparatus and gas-loops for calibration of gas monitors.

M. Yoshida et al. I Nucl. Instr. and Meth. in Phys. Res. A 383 (19961 44-446 443

(hereafter called 1500 cm3 ionization chamber) was select-

ed as an objective instrument to be calibrated because it

was popularly used not only for air-contamination mea-

surements but also as a secondary standard instrument in Japan [6,7,9]. The output ionization currents were mea- sured with a vibrating-reed electrometer which was cali-

brated with a standard current source. On the calibration of the 1500 cm’ ionization chamber,

air was first mixed with a radioactive gas in the gas loop including the ionization chamber. The gas mixture was

quantitatively introduced into the gas-sampling chamber of (48.81+0.04) cm3. According to the method mentioned in

the next section, the activity concentration of the sampled

gas mixture was then measured after being mixed with PlO gas in the loop for the proportional counter. The ambient

temperature was controlled at (222 1 )“C during the gas dispensation and activity concentration measurement. The

time interval between the ionization current and activity

concentration measurements was recorded for decay cot- rection and kept less than 10 min.

2.3. Activity measurement and calibration method

The gaseous activity injected into the proportional counter. A [Bq], can be expressed by [3,6]

A= Ro

(1 -Frh)(l -F,)(l -F,)’

where R, is the count rate in the proportional counter

[s- ‘1, Fth is the proportion of lower energy part of beta-ray spectrum than a threshold energy, F, is the proportion of count-loss due to wall effect. and F, is the proportion of

count-loss due to end effect. The term (I - F,) can be equal to unit in the DLPC

method. As both F,, and F, depend on a discrimination

level of detection system, the term (1 - F,,)( 1 - F,) of

Eq. (I ) can be rewritten as ( 1 - F,,,,). It is determined in each measurement on the basis of pulse height spectrum of

measured beta-rays and discrimination voltage.

The injected radioactive gas is uniformly mixed with the counting gas inside the gas loop for the proportional

counter after its activity is determined according to Eq. (1). Then, the ratio of the determined activity, A [Bq], to the count rate measured after mixing, r0 [s- ‘1, is obtained as k = A/r,. Since an electronegative gas of air reduces gas

multiplication, its introduction to the proportional counter leads to the decrease of count rate. Consequently, the activity concentration of the mixture of air and radioactive

gas in a gas monitor to be calibrated, C [Bq cme3], is given as

C= kr u(l - Fa)’

(2)

where r is the count rate on measurement of the gas mixture [s-l], v is the volume of gas-sampling chamber

[cm’], and F, presents the proportion of decrease of count

rate when air is mixed into the counting gas. The activity

concentration determined by this method depends on the

volume of gas-sampling chamber, but not on the volume of gas loops for calibration.

The term (1 - Fa) of Eq. (2) can be experimentally

given by means of mixing a known amount of air into the

counting gas and measuring the variation of count rate as a function of the amount of air.

3. Results and discussion

3.1. Purity of produced radioactive rare gases

Fig. 2 shows the gamma-ray spectra for the produced radioactive rare gases of “‘Ar and ‘35Xe. There are no

observable gamma-ray peaks other than those from the

objective radionuclide and 40K in the spectra measured 4

hr after the irradiation. The results show that the gases of

4’Ar and ‘j5Xe have high radiochemical purity enough to be used for calibration of gas monitors. On the other hand,

the gas of ‘j3Xe contains a considerable amount of

13’Xem, as seen in Table 1. The compositions of radionu- elides in the lZZXe gas were derived from the observed

peak areas for the corresponding gamma-ray energies,

_. ._. lOoi , , , , , ,

0 1000 200

t 5 5

io; (b)135Xe m 104; ix

> 1

2:

m :> >> .d i t dd

vrur

s . . . tzi pz . . . -__... ..-_- ._... :: f : : ..---. __--. -. _. _ . . _-_-._ 100, I , , , , , , , ,

0 500 1000

Gamma-ray energy ( keV )

Fig. 2. Gamma-ray spectra of prepared radioactive rare gases: (a) 4’Ar and (b) 13’Xe. They were measured 4 hr after irradiation.

444 M. Yoshida et al. / Nucl. Instr. and Meth. in Phw. Res. A 383 (1996) 44-446

which were 162 keV for ‘3’Xem, 81.0 keV for ls3Xe and 233 keV for ‘33Xem, respectively. Since the half-life of ‘j3Xem (2.19 d) is not so short in comparison with that of

17’Xe (5.24 d), the considerable activity of ‘33Xem still remains a few days after the irradiation. It is important to study the influence of lJ3Xem on calibration of gas monitors and to determine an optimum cooling time in consideration of its half-life and the yield of ‘33Xe activity.

3.2. Determination of activity concentration

In Table 2 are listed the measured values of k and Frhtw for 4’Ar, ‘33Xe and “’ Xe, accompanying those for “H, ‘? and “Kr [6]. The values of k change little for different radionuclides. The fluctuations in repeated measurements

were settled within their statistical uncertainties. The

correction concerning Fth+w is dominant for 3H with

extremely low maximum beta-ray energy or for 4’Ar

emitting the high-energy electrons with low stopping power. The threshold level of detection system was set

sufficiently low, which was determined corresponding to

the energy of L-Auger electron in the decay of 37Ar. The values of Ft,,+w therefore are small enough to contribute little uncertainty to the correction. Especially for ‘3Xe, the corrected value is less than 0.5% because of the additional

count of internal conversion and Auger electrons. In Fig. 3, the relations between the value of (1 - F=) and

the amount of air mixed into the counting gas are shown

for 4’Ar, 13’Xe and 13’Xe. The values of (1 - F,) moder-

ately decrease with increasing amount of air because the

pulse amplitude is reduced by the air mixing. Since the F, depends on the pulse height distribution, the variation of F, shows a similar tendency to that of Frh+w given in Table 2.

On the dispensation through the gas-sampling chamber, the partial pressure of air in the proportional counter is

normally about 2.5 kPa, compared with 98.1 kPa of the counting gas. At the partial pressure, the values of F, are

relatively small, not exceeding 5% for 4’Ar and 1% for

“‘Xe.

3.3. Calibration of 1500 cm-’ ionization chamber

Fig. 4 shows the ratios of ionization current to activity

Table 2

Measured values of k and F,,,, for each radionuclide

Nuclide

‘H

‘?I

wr “Ar

“‘Xe

“‘Xe

Maximum P-ray

energy [keV]

18.6(100%)

156(100%)

687(-99%) 1198( >99%)

346( >99%)

909(%%)

k [Bqsl

1.2066~0.0031

1.1882-tO.0034

1.1892+0.0033

1.1946+0.0020

1.1740+0.0022

1.1851-fO.0016

F ,hiW WI

1.6

0.6

1.2 1.7

0.3

1.0

0.96

2 & 0.96

Partial pressure of air ( kPa)

Fig. 3. Variation of (1 - F,) as a function of partial pressure of air.

'%Xe

:::::I,- "Ar 1 -. I I 1 I

80 100 120

Internal air pressure ( kPa )

Fig. 4. Variation of current per activity concentration vs. internal

air pressure for the 1500 cm3 ionization chamber.

concentration for 4JAr, “ZXe and ‘j5Xe, which were measured for the 1500 cm’ ionization chamber as a function of the internal air pressure. The total errors were estimated less than 5% in 99.7% probability, including the

influences of ambient temperature, the gas-tightness of

mixing loop and the stability of measuring system. The

ratios for ‘33Xe were measured with the ‘33Xe gases.

supplied by LMRI(France), with high radiochemical purity

(‘33Xem: 0.12%, 13’Xem: 0.52%). The ionization efficiencies derived from the data in Fig.

4 are plotted in Fig. 5, accompanying those for 14C and “Kr [6]. The mean electron energies in the horizontal axis

are defined by dividing the sum of energy over whole emitted electrons by their number. Monte Carlo calcula- tions with the EGS4 code [8] give the ionization efficien- cies for 4’Ar, ‘73Xe and ‘35Xe in the same figure [7]. The ionization efficiencies, as seen in Fig. 5, systematically and reasonably decrease with increasing mean electron energy. The measured values show good agreement with the

M. Yoshida et al. I Nucl. Insrr. and Meth. in Phys. Res. A 383 (1996) 441-446 445

I I L.I.1 I 102 lo3

Mean electron energy ( keV )

Fig. 5. Relation between ionization current and mean electron energy for the 1500 cm’ ionization chamber.

calculated ones. The ionization efficiencies of the same type of ionization chamber had been experimentally evalu- ated for three radionuclides of 4’Ar, “Kr and “‘Xe on the basis of the standards supplied by ETL, the national standard laboratory of Japan [9]. The standards were determined by a conventional internal gas counting method (length compensation method) [l]. Their results are also well consistent with the present ones. These consistencies

explain the validity and reliability of the present calibration

method.

3.4. Influence of impurity for ‘33Xe on calibration

Fig. 6 gives the ratios of ionization current to activity concentration for 13’Xe gases containing different amounts of Ix3Xern and 13’Xem, which are measured with the 1500 cm’ ionization chamber. The activity concentrations de-

i “‘““r-----T 7

m” 44 - a

.Q&e - 0 a

a

‘: 0.45 8 O’V A A :

I Ls

0 *’ De b .f

9!&.- &

‘Z= 8- tk

2 - _, $1 P‘;D; h 5 0.40- ‘“Xe” “‘Xe” -

a. 0 0.12% _j’ a a

0.52% _

E

. <O.Ol% 3.8% d 0 0.31% 18% -

k 0 1.3% < 0.4% _

3

a 17% < 0.7%

0.35 I I I I I 80 100 120

Internal air pressure ( kPa )

Fig. 6. Influence of “‘Xe” and “‘Xem on ionization current per activity concentration.

( ‘3’Xem/‘33Xe) 0

0 .

l

I 1,,,*,1 I I 103 104

Volume of ionization chamber ( cm3 )

Fig. 7. Relation between ionization current and effective volume of ionization chamber. The symbols of 0 and 0 show the

calculated values by the Monte Carlo method.

termined according to JZq. (2) were corrected with the measured composition of radioactive xenon and the emis- sion rates of internal conversion electron [5]. In Table 3, the ratios at an internal gas pressure of 101.3 kPa are compared with the corresponding values calculated with the EGS4 code. The measured and calculated values are quite consistent with each other while the former values are a few % higher than the latter ones.

In the case of monitors of a gas-flow ionization chamber type, the influence of impurities varies with the effective volume and shape of ionization chamber. Fig. 7 gives the variation of the ionization current per decay for the impurity nuclide( ‘33Xe”’ or ‘3’Xem) with the volume of ionization chamber, which is evaluated using the EGS4 code. In this calculation, the effective volume was changed in keeping a similar shape to the 1500 cm’ ionization chamber. The plot is shown of the ionization current for each nuclide divided by that for lA3Xe. In the volume range shown in Fig. 7, the ratio for “‘Xe” approaches to unit as the volume increases. The similar tendency is observed for “‘Xe”. Most of the gas monitors of ioniza- tion chamber type used for radiation protection purpose have a relatively large effective volume over several thousand cm3. In this range of effective volume, the influence of these impurities is expected to be much smaller than that for the 1500 cm3 ionization chamber.

4. Summary

A series of procedures from production of radioactive rare gases of 4’Ar, ‘33Xe and ‘35Xe to calibration of gas monitors with them was presented and discussed.

The activity concentrations of these gases were de-

446 M. Yoshida et al. I Nucl. Instr. and Meth. in Phys. Res. A 383 (1996) 441-446

Table 3 Comparison between measured and calculated values concerning the current per activity concentration of the 1500 cm’ ionization chamber for different compositions

Exp. no.

1

2

3

4

5

Composition [%]

Iz3Xe: 99.4 “‘Xe”: 0.12 “‘Xe”: 0.52 “‘Xe: 95.9 lq”Xem: 0

“‘Xe”: 4.1 “‘Xe: 98.3 ““Xe”: 1.3 “‘Xe”: 0.4 “‘Xe: 84.2 ‘jZXe”: <0.3 “‘Xe”: 15.2 ‘j’Xe: 85.0 13’Xem: 14.4 “‘Xe”: CO.6

Current/activity cont. [PA Bq-‘cm’]

Measurement(ratio) Calculation(rati0)

0.444( 1 .ooo) 0.453( 1.000)

0.443(0.998) 0.448(0.990)

0.438(0.986) 0.450(0.995)

0.433(0.973) 0.431(0.952)

0.424(0.954) 0.424(0.936)

termined on the basis of the DLPC method and quantita-

tive gas dispensation, and necessary correction factors for

the determination were estimated. The corrections can be

made easily and accurately, because their values are small enough and can be experimentally obtained.

The 1500 cm3 ionization chamber used as a secondary

standard was calibrated with small uncertainties for these three radionuclides. The results were well consistent with

those obtained by the Monte Carlo calculation and the previous experimental data. The validity and reliability of

the present method were demonstrated. On the calibration of the 1500 cm7 ionization chamber,

the influence of 13jXern and ‘3’Xem was examined with

gases of their different composition, and also evaluated by Monte Carlo calculations. In the case of highly enriched

“‘Xe used for a target gas, the influence of impurities is

not serious by taking a cooling time of one week in consideration of the yield of ‘33Xe.

[2] C. Moo, T. Yamamoto, T. Suzuki, A. Uritani, K. Yanagida, Y. Wu, T. Watanabe and M. Yoshida, Nucl. Instr. and Meth. A 312 (1992) 189.

[3] M. Yosbida, T. Yamamoto, Y. Wu, T. Aratani, A. Uritani and C. Mori, Nucl. Instr. and Meth. A 330 (1993) 158. Erratum, ibid. A 335 (1993) 583.

[4] J.S. Merritt, W.F. Merritt, L,V Smith and A.R. Rutledge, AECL-7427 (1981).

[5] National Council on Radiation Protection and Measure- ments, NCRP Report No. 58 (1985) p. 368.

[6] M. Yoshida, Y. Wu, Y. Ohi and T. Chida, Radioisotopes 42 (1993) 452 (in Japanese).

[7] T. Torii, Nucl. Instr. and Meth. A 356 (1995) 255. [8] W.R. Nelson, H. Hirayama and D.W.O. Rogers, SLAC

Report-265 (1985). [9] Nuclear Safety Research Association (NSRA), in: Study on

the measurement of Environmental Radiation and Radioac- tivity (1980) p. 125 (in Japanese).

References

[I] W.B. Mann, H.H. Seliger, W.F. Marlowand and R.W. Med- lock, Rev. Sci. Instr. 3 1 (1960) 690.