Dependence of NaI(Tl) detector intrinsic efficiency

on source--detector distance, energy and off-axis

distance: Their implications for radioactivity

measurements

F O OGUNDARE^1,*, E O ONIYA² and F A BALOGUN³

¹Department of Physics, University of Ibadan, Ibadan, Nigeria

²Department of Physics and Electronics, Adekunle Ajasin University,

Akungba-Akoko, Ondo State, Nigeria

³Centre for Energy Research and Development, Obafemi Awolowo

University, Ile-Ife, Nigeria

^*Corresponding author. E-mail: ogun\_dare@yahoo.com

Abstract. In this work the dependence of intrinsic efficiency of a

NaI(Tl) detector of radius 3.82 cm and height 7.62 cm on

source--detector distance ($d$), source--off-axis distance ($d_{0}$)

and $\gamma$-photon energy have been investigated using analytical and

Monte Carlo methods. The results showed that, for a given off-axis

distance, there exists a value of the ratio of source--detector

distance ($d$) to detector radius ($R)$ where intrinsic efficiency is

minimum. This $d/R$ value at which minimum efficiency occurs

approaches zero as off-axis distance increases and it is almost

constant with increase in energy. In the region where $d/R < 0.01$, a

criteria given by Jehouani {\it et al} [1] for good photon detection,

intrinsic efficiency decreases with increasing off-axis distance. The

implications of the results for radioactivity measurement and

radiation protection are discussed. Chacteristics of intrinsic

efficiency in the regions $d/R < 0.01$ and $d/R >$ 10 are also

compared.

Keywords. Detections; $\gamma$-photons; analytical formula; Monte Carlo; intrinsic

efficiency.

PACS Nos 29.40.-n; 29.40.Mc; 21.60.Ka