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Journal of Analytical Sciences, Methods and Instrumentation, 2018, 8, 25-36

http://www.scirp.org/journal/jasmi

ISSN Online: 2164-2753

ISSN Print: 2164-2745

DOI:

10.4236/jasmi.2018.83003 Sep. 7, 2018 25 Journal of Analytical Sciences, Methods and Instrumentation

Performance Evaluation of the Thick Pinhole

Gamma Rays Diagnostic System

Hongwei Xie, Jinchuan Chen, Linbo Li, Qiang Yi

Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang, China

Abstract

The diagnostic methods for the profile of the radiation source were estab-

lished at first based on the pinhole imaging principle. In this paper, the rela-

tionships among various parameters of the gamma-

rays crammer such as the

modulation transfer function (

MTF

), the noise power spectrum (NPS),

the

signal-noise ratio (

SNR

) and the detective quantum efficiency (DQE) are de-

veloped and studied experimentally on the cobalt radiation source. The image

diagnostic system is consisting with rays-fluorescence convertor (YAG crys-

tal), optical imaging system,

MCP

image intensifier,

CCD

camera and other

devices. The spatial resolution of the modulation transfer function (

MTF

) at

10% intensity was measured as 1 lp/mm by knife-

edge method. The quantum

of the measurement system is about 150 under weak radiation condition due

to the single particle detection efficiency of the system. The dynamic range

was inferred preliminarily as about 437. The required radiation intensity was

calculated using the experiment result for the

SNR

= 1, 5, 10,

respectively.

The theoretical investigation results show that the radiation image with

SNR

= 1 can be only obtained when the pinhole diameter is 0.7 mm, object dis-

tance and image distance are both 200 cm, and the radiation intensity is about

1.0 × 10

−1

⋅cm

−2

Keywords

Gamma-Ray Camera, Pinhole, YAG,

MCP

Image Intensifier,

CCD

MTF

NNPS

1. Introduction

The

(neutron)-rays camera based on the pinhole imaging principle are widely

*1) Sponsered by: National Natural Science Foundation of China (Contract No. 11005095,

11305155). 2) About the author: Hongwei Xie (1966), male, associate professor, master of science

mainly engaged in the study of image diagnostics of pulse radiation source.

How to cite this paper:

Xie, H.W., Chen,

.C., Li, L.B. and Yi, Q. (2018)

Performance

Evaluation of the Thick Pinhole Gamma

Rays Diagnostic System

Journal of Analy

ical Sciences

Methods and Instrument

tion

, 25-36.

https://doi.org/10.4236/jasmi.2018.83003

Received:

May 9, 2018

Accepted:

September 4, 2018

Published:

September 7, 2018

8 by authors and

Scientific

Research Publishing Inc.

This work is licensed under the Creative

Commons Attribution International

License (CC BY

4.0).

http://creativecommons.org/licenses/by/4.0/

Open Access

H. W. Xie et al.

DOI:

10.4236/jasmi.2018.83003 26 Journal of Analytical Sciences, Methods and Instrumentation

used in the diagnostics of the radiation sources [1]. The diagnostic method based

MCP

image intensifier, due to its performance in image intensification and

shutter time gating, is also widely used in the study of imaging diagnostics of the

pulse radiation source and weak radiation sources [2] [3]. The system consists of

rays-fluorescence convertor (YAG crystal), optical imaging system,

MCP

image

intensifier,

CCD

camera and other devices. Radiation imaging quality is not only

dependent on the quality of the imaging system, but also on other various fac-

tors, e.g., the incident flux of the

-rays, the detecting efficiency of the

-rays,

energy conversion efficiency and quantum gain of the photographic system, etc.

On the other hand, imaging based on radiation source combined with thick

pinhole is an ideal method of nondestructive detection with high spatial resolu-

tion. Point radiation source is ideal to be used in the nondestructive detection as

the size of the radiation source is the critical factor for the spatial resolution,

while it’s challenging to decrease the source size during the implementation of

the system hardware. However the spatial resolution of the thick pinhole imag-

ing is determined by the aperture of the pinhole, which can be reduced to im-

prove the spatial resolution. In addition, this method for nondestructive detec-

tion will not be restricted by the source size.

Generally, performance of the imaging diagnostic system is calibrated with

three main technical specifications: the modulation transfer function (

MTF

) [4],

the noise power spectrum (NPS) [5] [6], and the detective quantum efficiency

(DQE) [7] [8].

MTF

is a universally accepted standard to calibrate the spatial

resolution of the system. NPS, reflecting of the effects of the noise on the image

quality, could present the difference of the image quality under different condi-

tions of frequency spectra. DQE is used to reflect the detecting efficiency of the

imaging diagnostic system to the

-ray transmittance from the radiation source.

Among the above-mentioned three specifications, there should be certain inhe-

rent relationships. In the case of the diagnostics of the high energy radiation

sources, due to the relatively intense penetration effects of the

-rays, the edge

method becomes the major method to determine the

MTF

, which is represented

by the Fourier Transform of the linear spread function (LSF). As for the NPS, it

is resulted from the Fourier Transform of the image under the conditions of the

flat field effects. The noise power spectra are different under different irradiation

conditions. Another performance of the NPS is that it could be used to provide

other parameters such as the signal-noise ratio (

SNR

). Finally in dealing with the

DQE, it is related with such parameters as the detecting efficiency and quantum

gain during the signal transmission [9]. The fluorescence gain of the scintillator

is more than 10

photon/MeV [10]. At the same time, the quantum gain of the

MCP

imaging intensifier is also higher than 10

photon/photon. Thus, the

quantum gain becomes an important factor affecting the image quality of the

high-gain imaging diagnostic system.

In this paper, relationships among various parameters of the gamma-rays

camera,

MTF

, NPS and DOE, are researched based on thorough analysis of the

H. W. Xie et al.

DOI:

10.4236/jasmi.2018.83003 27

Journal of Analytical Sciences, Methods and Instrumentation

-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

-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

-rays. YAG crystal is used to convert the

-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

-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

-rays direction to

reflect the fluorescence image, as well to avoid direct

-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

-rays in the scintillator is Φ, and that the

interactions of the

-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

SNR = Φ

. 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

-rays and the scintillator. Due to the

fact that the

-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

-rays could

be given in the following equation [11]:

( )

exp

π 1

φµ



Φ= ⋅ ⋅ ⋅ − −





(1)

where:

is the output intensity of the

-rays from the radiation source;

is the

pinhole diameter;

is the object distance,

is the amplification factor;

is the

scintillator thickness;

is scintillator linear attenuation coefficient. Substitute the

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