富山大学研究推進機構水素同位体科学研究センター研究報告 36：21－29，2016.

論文

光電子増倍管の印加電圧を制御する改良積分法の液体シンチレーション計測への応用
原正憲 a，坂口

春菜 a，中山将人 a，阿蘇司 b，庄司

古澤孝良 d，加藤結花 d，吉村共之 d

富山大学

研究推進機構

水素同位体科学研究センター

a

美樹 c，

〒930-8555 富山県富山市五福 3190b

富山高等専門学校

電子情報子工学科〒939-0293 射水市海老江練合c

富山大学

研究推進機構研究推進総合支援センター

1-2
生命科学先端研究支援ユニット〒930-0194 富山市杉谷 2630d

日立製作所〒181-8622 東京都三鷹市牟礼6-22-1

Modified integral counting method by controlling high voltage ofphotomultipliers for liquid scintillation counting
Masanori Haraa, Haruna Sakaguchia, Masato Nakayamaa, Tsukasa Asob, Miki Shojic,Takayoshi Furusawad, Yuka Katod, Tomoyuki YoshimuradaHydrogen Isotope Research Renter, Organization for Promotion of Research,
University of Toyama, Gofuku 3190,Toyama 930-8555, Japanb

Electronics and Computer Engineering, Toyama National College of Technology,
Ebie-neriya 1-2, Imizu city, Toyama 933-0293, Japanc

Life Science Research Center, Organization for Promotion of Research, University ofToyama, Sugitani 2630, Toyama 930-0194, JapandHitachi, Ltd.,

6-22-1 Mure, Mitaka city, Tokyo 181-8622, Japan(Received January 27, 2017; accepted July 24, 2017)21原正憲・坂口春菜・中山将人・阿蘇司・庄司美樹・古澤孝良・加藤結花・吉村共之

AbstractThe modified integral counting method by control of the high voltage ofphotomultipliers (MICM-HV) was investigated to show its applicability forradioactivity analysis. In the MICM-HV, pulse height spectra of the sample aremeasured at various high voltages of photomultipliers in the liquid scintillation counter.
The spectra are converted to integral spectra, which are extrapolated to give theconvergence point. The counting rate at the convergence point corresponds to thedisintegration rate of the sample. The MICM-HV determines the disintegration rate with
one cocktail sample and the method requires no unquenched standard sample.
Keywords: Modified integral counting method, high voltage, photomultiplier,scintillator

1. Introduction
Liquid scintillation counting (LSC) is widely used for the radioactivity analysis
of beta emitters, whereas LSC requires a quenching correction to determine a counting
efficiency. Several quenching correction methods have been proposed. The quenchingcorrection is usually carried out using a quench correction curve which is made by aquenched standard set [1]. Quenched standard sets for both 3H and 14C can be purchasedfrom some suppliers, whereas standards sets for other nuclides need to be prepared byoneself. On the other hand, no quenched standard is required in direct methods [1] suchas the integral counting method [2] and the efficiency tracing method [3]. Homma et al.
have proposed the modified integral counting method (MICM) [4, 5]. The MICM has
been used routine works of moderate precision, because no complicated procedure isnecessary. In this method, the disintegration rate is obtained from the integral count rateof the sample at the zero detection threshold (ZDT). However, 3H unquenched standard
sample which the activity of 3H is known is required to determine the ZDT. The pulse
height spectrum of 3H unquenched sample is measured, and the spectrum obtained istransformed to the integral spectrum,
Chmax

IC (Chi )   C (Ch)Ch Chi22(1)光電子増倍管の印加電圧を制御する改良積分法の液体シンチレーション計測への応用

where IC(Chi) is the integral counts from a given integral channel (Chi) to themaximum channel (Chmax) and C(Ch) is the count rate at a given channel number,respectively. The integral spectrum is extrapolated to the disintegration rate of thestandard sample and the ZDT is found. Subsequently, the pulse height spectrum of an
unknown sample is measured and it is converted to the integral spectrum by Eq. (1).
The integral spectrum of the sample is extrapolated to the ZDT, the disintegration rateof unknown sample is determined the count rate at the ZDT. To know the disintegration
rate of unknown sample the MICM requires an unquenched standard of 3H. The MICM
with various quenched samples (MICM-VQ) requires several quenched samples having
same amount of radioactive material[6]. In the MICM-VQ these spectra are converted
to integral spectra by Eq. (1) and these integral spectra are extrapolated to find theconvergence point which corresponds to the ZDT, because the integral counts at theZDT does not depends on the quenching of the sample. The integral count rate at theZDT corresponds to the disintegration rate of sample. However, this technique can not
be applied for one sample.
In this study, the determination method of the disintegration rate of one sample
cocktail without an unquenched standard was developed. Several integral spectra of one
sample were obtained by the control of the high voltage of photomultipliers (PMTs).
These spectra were extrapolated to find the ZDT like as the MICM-VQ. The
disintegration rate of the sample was obtained from the integral counts at the ZDT. The
method developed was named as the modified integral counting method by the controlof high voltage of PMTs (MICM-HV). The disintegration rates obtained by theMICM-HV were compared with those by the MICM-VQ.

2. Experimental

The quenched standard set of 14C of toluene based scintillator (denoted as TLset) and that of Ultima Gold (denoted as UG set) were purchased as sample cocktails.
The quenched standard sets were composed of 10 samples which contained sameamount of radioactive material. The disintegration rate of TL set and of UG set weredetermined to be 128700 dpm and 122200 dpm by suppliers, respectively. The23

原正憲・坂口春菜・中山将人・阿蘇司・庄司美樹・古澤孝良・加藤結花・吉村共之

uncertainty of both sets was ±1.3% in 99% confidence level. The five samples of TL set
are named from TL A to TL E with increasing quenching, and the samples of UG set arefrom UG 1 to UG 5.
The pulse height spectra from scintillation cocktails were measured by theliquid scintillation counter, Aloka LSC-LB5. This scintillation counter has three ofPMTs for the scintillation counting with the coincidence unit to reduce the backgroundcounts. This system is equipped the 4000 channel multichannel analyzer to register thepulse height spectrum of scintillation cocktail. The channel range for measuring the 14C
spectrum was adjusted from 0 to 200 keV. The pulse height spectra of samples wereregistered for 10 min when the high voltage of PMTs was controlled in the range from1500 V to 1540 V.

3. Results and discussion
3.1. MICM with various quenched samples
Nakayama et al. reports MICM-VQ can be applied for various liquid
scintillators such as toluene and Ultima Gold [7]. The disintegration rates of TL and UG
sets were determined by the MICM-VQ. Fig. 1 shows the pulse height spectra of UG set
at 1533 V for PMTs. The abscissa
and the ordinate are the channel
number and the counting rate,respectively. The pulse height of
UG 1 with the lowest quenchinglevel is extended to 2500 channel.
However, the pulse height ofquenched samples decreased withincreasing the quenching. On theother hand, the peak positions ofpulse height spectra were kept.

spectra were obtained under 1533V of the high

These spectra were converted tovoltage of photomultipliers in LSC-LB 5.

the integral spectra with using

24Figure 1. Pulse height spectra of UG set. These光電子増倍管の印加電圧を制御する改良積分法の液体シンチレーション計測への応用

Eq. (1). They are shown in Fig. 2.
The value of IC decreased withincreasing

Chi.

The

fittingfunction was obtained from theintegral spectrum where a second
order function was applied. The
fitting was done for the higher
channel above the peak around150

channelwhenthehighchannel was set to be satisfied

Figure 2. The integral spectra of UG set. The integral

R 2  0.9997 . The convergence

spectra were obtained from the pulse height spectra

point was found at -13 channelwith using Eq. (1). The high voltage of PMTs wasand the disintegration rate ofadjusted at 1533 V. The bold and thin lines indicate

samples was evaluated to be
123.8 ±0.2 kdpm. Since low

theobtainedspectra

andthefitting

curves,respectively.

energy beta particles can generate insufficient number of photons to detect, a channel
number of a convergence point has a negative value. The uncertainty of obtained value
is the standard deviation of fitting functions at the convergence point. The counting rate
at the convergence point agreed with the assayed value of UG set. The evaluations of
the disintegration rate of both TL and UG sets at other values of high voltage of PMTs
were listed in Table 1 and 2, respectively. The disintegration rate evaluated agreed withthe assayed value by the supplier. Within the measurement range the high voltage ofPMTs does not influence on the evaluation of the disintegration rate by the MICM-VQ.
Table 1. Obtained value of Toluene quenched set (TL) with MICM-VQ.
High voltage (V)Convergence

Disintegration rate

[obtained]/[assayed]

point (Chi)(kdpm)

1500-1128.4±0.3

1.00

1510-2

128.4±0.41.00

1520-1128.2±0.4

1.00

1533-3128.1±0.3

1.00

1540

1

127.3±0.2

0.99

25

原正憲・坂口春菜・中山将人・阿蘇司・庄司美樹・古澤孝良・加藤結花・吉村共之

Table 2. Obtained value of Ultima Gold quenched set (UG) with MICM-VQ.
High voltage (V)Convergence

Disintegration rate

[obtained]/[assayed]

point (Chi)(kdpm)

1500-15

124.2±0.3

1.02

1510

-13

123.9±0.2

1.01

1520

-13

123.9±0.1

1.01

1533

-13

123.8±0.2

1.01

1540

-12

123.4±0.2

1.01

3.2. MICM by control of high voltage of PMTs
The MICM-VQ requires several samples, which contain same amount ofradioactive material, to determine the ZDT. If the several spectra are obtained from one
sample, the disintegration rate of its sample will be determined by using the MICM. By
controlling the high voltage of PMTs, the pulse height from PMTs can be regulated. The
control of high voltages works like an addition of quencher. Therefore, several pulse
height spectra can be obtained from one sample by the control of the high voltages. To
confirm the applicability of the MICM by the control of the high voltage of PMTs(MICM-HV), the pulse height spectra of both TL and UG sets were obtained under
several different values of high voltage.
Fig. 3 shows the dependence of pulse height spectra on the high voltages ofPMTs. These spectra wereobtained from UG1 whichwasa

lessquenchedsample in UG set. Thepulse height spectra werestretched to the higher
pulse

heightsidewith

increasing high voltage of
the PMTs. However, thepeak position of the pulse
heightspectra

maintained around 150

26

wasFigure 3. Pulse height spectra of UG 1 at various high voltagesof PMTs in LSC-LB5.

光電子増倍管の印加電圧を制御する改良積分法の液体シンチレーション計測への応用

channel. This change inthespectrawiththevariation of high voltages
was similar to the effect ofthe quenching as shown inFig. 1. In other words, thecontrol of the high voltage
of PMTs can provide a setof spectra like a set ofquenched samples. Thesespectra

obtained

wereconverted to the integral
spectra with using Eq. (1)Figure 4. The integral spectra of UG 1 at various high voltagesof PMTs. The bold and thin lines indicate the obtained spectra
and the fitting curves, respectively.

as shown in Fig. 4. All
integral spectra monotonically decreases. These spectra were fitted by a second orderfunction. The fitting curves from the integral spectra obviously show the convergencepoint at 7 channel. The counting rate at the convergence point was found to be 121.2
Table 3. Obtained value of Toluene quenched set with MICM-HV.
SampleConvergence

Disintegration rate

[obtained]/[assayed]

point (ch.)(kdpm)TL A

35

124.6±0.1

0.97

TL B

35

124.2±0.20.97

TL C

30

124.0±0.30.96

TL D

21

124.3±0.20.97

TL E

9125.4±0.2

0.97

Table 4. Obtained value of Ultima Gold quenched set with MICM-HV.
SampleConvergence

Disintegration rate

[obtained]/[assayed]

point (ch.)(kdpm)

UG 1

7

121.2±0.1

0.99

UG 2

-23

125.3±0.2

1.03

UG 3

6

120.3±0.2

0.98

UG 4-9123.0±0.2

1.01

UG 5

-15

124.3±0.2

1.02

27

原正憲・坂口春菜・中山将人・阿蘇司・庄司美樹・古澤孝良・加藤結花・吉村共之

kcpm. The integral spectra of both TL and UG sets were obtained, and the disintegration
rates were evaluated from the convergence point. These results of TL and UG werelisted in Tables 3 and 4, respectively. In TL set, the disintegration rates evaluated were
3 % less than the assayed value. The values evaluated for UG set slightly deviated fromthe assayed value within ±3 %. These results indicate that the MICM-HV can be used
for the radioactivity measurement of moderate precision. It is also worth to mention thatthe MICM-HV is able to determine the disintegration rate from only one sample and it
was applicable for various scintillators.

4. Conclusions

A new method based on the MICM, MICM-HV, was developed for themeasurement of disintegration rate of a single sample without unquenched standard.
The pulse height spectra of a sample were measured under various high voltages ofPMTs in the commercial liquid scintillation counter. The pulse height spectra obtained
were converted to the integral spectra to find the convergence point, and thedisintegration rate was determined from the integral counts at the convergence point.
The disintegration rate evaluated by the MICM-HV agreed with the assayed value
within 3 % of deviation. It indicates that the MICM-HV has a potential for theradioactivity analysis from one sample with moderate precision.

Acknowledgement

This work was supported by JSPS KAKENHI Grant Number 26610065.

References[1] M.F. L'Annunziata, M. J. Kessler, Liquid scintillation analysis: principles andpractice, Chpt. 5, M. F. L'Annunziata (Eds.), Handbook of Radioactivity Analysis.
Academic press, California, (2003).
[2] G. Goldstein, Absolute liquid-scintillation counting of beta emitters., Nucleonics

28

光電子増倍管の印加電圧を制御する改良積分法の液体シンチレーション計測への応用

23(1965) 67-69.
[3] M. Takiue, H. Ishikawa, Thermal neutron reaction cross section measurements forfourteen nuclides with a liquid scintillation spectrometer. Nucl. Instrum. Methods.,
148(1978) 157-161.
[4] Y. Homma, Y. Murase, K. Hanada, The zero detection threshold of a liquidscintillation spectrometer and its application to liquid scintillation counting, Appl.
Radiat. Isot., 45(1994) 341-344.
[5] Y. Homma, Y. Murase, K. Hanada, Absolute liquid scintillation counting of 35S and45Ca using a modified integral counting method, J. Radioanal. Nucl. Chem., 187(1994)
367-374.
[6] M. Hara, M. Nakayama, K. Hirokami, T. Aso, Appropriate quenching level inmodified integral counting method by liquid scintillation counting, J. Radioanal. Nucl.
Chem., 310(2016)857-863.
[7] M. Nakayama, M. Hara, M. Matsuyama, K. Hirokami, Modified counting methodwith various quenched samples for different scintillators, Radio. Safety Manage.,16(2017)1-7.

29