Gamma-ray shield that can be evacuated, refilled with a clean gas, and pressurized for exclusion of airborne radioactive contaminants effectively lowers background noise. Under working conditions, repeated evacuation and filling procedures have not adversely affected the sensitivity and resolution of the crystal detector

Brar, S. S.

Gustafson, P. F.

Nelson, D. M.

English

NASA Technical Reports Server

August 1969	 Brief 69-10344 
. Icy
AEC-NASA TECH BRIEF 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
New Shield for Gal 
The problem: 
Effective shielding of a gamma-ray crystal asseni-
bly from background radiation due to cosmic rays or 
man-made radionuclides, as well as radiation from 
accelerators or x-ray machines located near the de-
tector. At present this background is reduced by 
shielding of both the detector and the sample by iron, 
lead, mercury, or other shielding materials that are 
essentially free of radioactivity. Such shielding, how-
ever, does not eliminate the background component 
due to airborne radionuclides present within the 
shield. 
The solution: 
A gamma-ray shield that can be evacuated, re-
filled with a "clean" gas, and pressurized for exclu-
sion of airborne radioactive contaminants effectively 
lowers background noise (1). Under working condi-
tions, repeated evacuation and filling procedures have 
not adversely affected the Nal(Tl) crystal detector; 
its resolution and sensitivity remain unchanged. The 
crystal used was jacketed with 3/32-inch stainless 
steel, and the evacuations were carried out to 0.5 
atm. The crystal temperature, inside the shield, comes 
to a rapid equilibrium after the shield has been evacu-
ated several times and pressurized with carbon di-
oxide.
rnma-Ray Spectrometry 
The shield's unique feature is that it can be evacu-
ated and filled with any desired gas. To make this 
possible the box is welded from the outside, and the 
door has a rubber gasket that seals against a tongue 
on the door opening. There are three access holes 
but none has exterior-to-interior straight-line paths. 
Two serve as filling and evacuation holes; the third 
provides access for electrical connections to the de-
tector. For low-radioactivity measurements the shield 
is evacuated and is filled from a pressure tank with 
a "clean" gas such as carbon dioxide—aged air; it is 
then pressurized slightly above ambient pressure. 
In practice the sample is introduced and the shield's 
door is closed. The shield is evacuated rapidly to 
about 0.75 or 0.5 atm, and the door is more firmly 
secured to the main body by a large threaded bolt 
that prevents the door from moving back and break-
ing the seal while the shield is being filled with gas. 
This procedure is repeated several times so that only 
a small portion of the original air remains. Since 
the crystal-jacket temperature changes less than 2°C 
during this procedure, an experiment can be started 
10 minutes later after the shield has come close to 
equilibrium temperature. 
Such a system has operated for more than 2 years 
with excellent results. 
How it's done: 
The gamma-ray shield consists of a rectangular 
box measuring (outside) 48 by 42 by 36 inches. The 
walls, 6 inches thick, are constructed from 2-inch-
thick, radioactivity-free, hot-rolled steel plates. The 
shield is lined with ½-inch lead and 1/16-inch 
cadmium and copper; this lamination tends to re-
duce the low-energy continuum and minimizes the 
effect of x-rays produced in the lead.
Reference: 
1. S. S. Brar, D. M. Nelson, and P. F. Gustafson, 
Intern. J. App!. Radiation isotopes 18, 261 
(1967). 
Notes: 
1. This information may interest gamma-ray spec- 
trosconists -	
-	 (continued 
This document was prepared under the sponsorship-of the Atomic 
Energy Commission and/or the National Aeronautics and Space 
Administration. Neither the United States Government nor any per-
son acting on behalf of the United States Government assumes any
liability resulting from the use of the information contained in this 
document, or warrants that the use of any information, apparatus, 
method, or process disclosed in this document may not infringe pri-
vately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000343 2020-03-16T18:12:30+00:00Z
2. Inquiries may be directed to: 
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10344 
Source: S. S. Brar, D. M. Nelson, and
P. F. Gustafson
Biological and Medical. Research Division
(ARG-10388)
Patent status: 
Inquiries concerning rights for commercial use of 
this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439 
Brief 69-10344	 Category 02


New shield for gamma-ray spectrometry

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