H. W. Xie et al.
10.4236/jasmi.2018.83003 27
Journal of Analytical Sciences, Methods and Instrumentation
γ
-rays camera. Meanwhile, the effects of these parameters upon the image quali-
ty are evaluated and studied experimentally.
2. Basic Principles of Pinhole Imaging with Gamma Rays
The schematic diagram of the basic principle of the thick pinhole imaging with
the
γ
-rays is given in Figure 1. The process could roughly be interpreted as fol-
lows. Firstly, the radiation source is imaged onto the imaging plane (YAG) by
γ
-rays. YAG crystal is used to convert the
γ
-rays into the fluorescence image.
Then the fluorescence image is imaged onto the incident plane of the
MCP
im-
age intensifier through the optical imaging system and conversion system. Fi-
nally, the intensified image is recorded by
CCD
device, and the
MCP
and
CCD
are coupled with fiber plate. Thus, the major two parts of the system are
γ
-rays
imagines system and the image recording system. Besides, a copper reflector
with an efficiency of over 95% is placed in a 45˚ angle with
γ
-rays direction to
reflect the fluorescence image, as well to avoid direct
γ
-rays irradiation onto the
MCP
image intensifier and
CCD
camera. The specially developed imaging sys-
tem could provide an amplification factor of 5:1 and a light collection efficiency
of over 95%. The
MCP
image intensifier was manufactured by Proxitronic Co.
Ltd. with a spatial resolution of over 37 lp/mm. The amplification factor of the
fiber plate used to couple the
MCP
and
CCD
is 1.5:1, providing a stable and re-
liable recording system.
3. Theoretical Modeling
3.1. Pinhole Imaging System
Suppose the detection efficiency of the
γ
-rays in the scintillator is Φ, and that the
interactions of the
γ
-rays and the scintillator are abiding by the Gaussian distri-
bution, then the fluctuated noise distribution induced by the detection efficiency
would be
, and accordingly the
SNR
of the signal input would be
. Another major factor to affect the quantum efficiency is the com-
monly called Swank coefficient, the energy distribution of the secondary elec-
trons induced by the interactions of the
γ
-rays and the scintillator. Due to the
fact that the
γ
-rays camera used in the experiment has a high gain and good de-
tection efficiency for single particle, the effects of Swank coefficient on the image
quality was excluded during our data analysis.
According to the pinhole imaging principles, the detection efficiency on the
scintillator imaging plane after the thick pinhole imaging with the
γ
-rays could
be given in the following equation [11]:
( )
( )
2
2
exp
π 1
1
4
D
t
L
M
φµ
Φ= ⋅ ⋅ ⋅ − −
(1)
where:
φ
is the output intensity of the
γ
-rays from the radiation source;
D
is the
pinhole diameter;
L
is the object distance,
M
is the amplification factor;
t
is the
scintillator thickness;
μ
is scintillator linear attenuation coefficient. Substitute the