**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 ONIYA^{2} and F A
BALOGUN^{3}

^{1}Department of Physics,

^{2}Department of Physics and Electronics,

Akungba-Akoko,

^{3}Centre for Energy Research and Development, Obafemi Awolowo

University,

^{*}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

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;

efficiency.

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