死时间测量新方法

Nuclear Instruments and Methods in Physics Research A 551(2005)352–355

A method of dead time measurement

Chul-Young Yi a,Ã,Keeju Jeong b ,Jang-Jin Oh c

a

Ionizing Radiation Group,Korea Research Institute of Standards and Science,P.O.Box 102,Yuseong,Daejon 305-600,Korea b

Department of Physics Education,Kongju National University,Sinkwan-dong 182,Kongju,Chungnam 314-701,Korea

c

Department of Radiation Protection,Radiation Safety Center,Korea Institute of Nuclear Safety,P.O.Box 114,Yuseong,Daejon 305-600,Korea

Received 10December 2004;received in revised form 30March 2005;accepted 8June 2005

Available online 13July 2005

Abstract

A practical method for the dead time measurement of a counting system is proposed.The method is based on the irradiation of a counting system in the reference photon field for the calibration of dosimeters or dose rate ing the method,we measured the dead time of a GM counting system.r 2005Elsevier B.V.All rights reserved.

PACS:07.85.Àm;29.40.Àn

Keywords:Radiation detector;Radiation counter;GM counter;Dead time;Dead time measurement

1.Introduction

In most detection systems of radiation,there is a certain amount of time required to separate two events in order that they be recorded as two different events,which is called the resolution time or dead time of a counting system.The dead time of a counting system affects inevitably the count-ing statistics.Even though two or more events occur,they are recorded as a single event if they come in the time interval shorter than the dead

time and,resultantly,the counting rate is mea-sured lower than the true rate.Because of the random nature of radioactive decay,there is always some probability that a true event will be lost,since it may follow too quickly the preceding event.The dead time correction is one of the essential elements in the accurate counting mea-surement of radiation.

In order to make dead time corrections,prior knowledge of the dead time is required.Sometimes this dead time can be associated with a known limiting property of the counting system (e.g.,a fixed resolving time or pulse shaping time of an electronic circuit).More often,the dead time will not be known or may vary with operating

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0168-9002/$-see front matter r 2005Elsevier B.V.All rights reserved.doi:10.1016/j.nima.2005.06.053

ÃCorresponding author.Tel.:+82428685370;

fax:+82428685671.

E-mail address:cyyi@kriss.re.kr (C.-Y.Yi).

conditions and must therefore be measured directly.

The common example of dead time measure-ment is the decaying source method[1,2].The method is practical when a radioisotope with a short half-life is available.The departure of the observed counting rate from the known exponen-tial decay of the source can be used to determine the dead time.The short-lived radioisotope is obtainable from neutron irradiation facilities but only a few laboratories can get a timely access to neutron irradiation facilities.Vinagre and Conde [3]suggest a method for the measurement of effective dead time of a counting system,which was based on the artificial piling-up of the detector pulses with electronic pulses delayed by a specific time interval.The method requires extra instru-mentation and the effect of the extra instrumenta-tion on the dead time measurements should be analyzed.

In the present paper we have proposed an efficient method for the dead time measurement of a counting system.The method is based on the irradiation of a counting system in the reference radiationfield for the calibration of dosimeters or dose rate meters.The variation of counting rate per unit dose was used to evaluate the dead time.

2.Method

Dead time corrections based on paralyzable and non-paralyzable models have been studied exten-sively and well summarized in Refs.[4–8].In the non-paralyzable model,the relationship between the true and observed counting rates is given by

n¼

m

1Àm t

(1)

where n is the true counting rate,m is the observed counting rate and t is the dead time.

The true rate of rays passing through a point in the reference radiationfield for calibrating dose rate meters is proportional to the dose rate at the point:

n¼kD(2)where n is the true rate,k is the proportionality constant and D is the dose rate at the point. Substituting Eq.(2)into Eq.(1),we have

m

D

¼kÀkm t.(3) If we take the abscissa as m and the ordinate as m=D,then Eq.(3)is a straight line having a slope ofÀk t and the intercept of k on the ordinate axis. In the paralyzable model,the relationship between the true and observed counting rates is given by

m¼n expðÀn tÞ.(4) Again,inserting Eq.(2)into Eq.(4),we have the equation

ln

m

D

¼ln kÀkD t.(5) Taking the abscissa as D and the ordinate as lnðm=DÞ,we have another straight line.This time, the slope is alsoÀk t whereas the intercept is ln k. The dead time can be deduced from the slope and the intercept on the ordinate axis.

3.Result and discussion

We applied the present method to determine the dead time of a GM counting system,which has been developed as a prototype alarm dose rate meter(ADR).We have not been involved in the development of the prototype ADR and the detailed information on the electronics of the prototype ADR has not been given.It was stated that the bias voltage of the GM tube was550V. The signal processing RC networks were designed as recommended by the manufacturer of the GM tube.Pulses generated in the GM tube were compared with a reference voltage by a compara-tor.A square output pulse was generated from the comparator when the height of the GM tube pulse was greater than the reference voltage.The square pulse was counted in the counting module.The prototype ADR was originally designed to convert the counting rate into the dose rate,but the specimen we have tested was programmed to transmit the counting rates to a PC via the serial RS-232port.

C.-Y.Yi et al./Nuclear Instruments and Methods in Physics Research A551(2005)352–355353