Low scatter, lightweight fission spectrometer provides a simple, reliable method for determining absolute neutron fluxes in a fixed neutron. It minimizes neutron scatter and energy degradation effects, and has a counting volume large enough to intercept the most energetic fission fragments, yet small enough to be discriminating

Frigerio, N. A.

English

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June 1968	 Brief 68-10174 
	
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	 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. 
Low Scatter Lightweight Fission Spectrometer 
Constructed for Biological Research 
Can Cover	 Fission Foil 
Can Bottom	 LdIIU LW 
	
Insulator	 Gas Inlet 
The problem: 
To construct a small, lightweight spectrometer for 
measuring the absolute neutron flux in a fixed neutron 
spectrum. Existing units are too massive to substitute 
for small biological objects suspended in a neutron 
beam. Studies of fission count rate, as a function of 
counter mass and composition, have shown that 
nearly complete elimination of hydrogenous materials 
and a total mass of less than 200 g are required to 
avoid undesirable neutron scatter and energy degrada-
tion effects. 
The solution: 
A low scatter, lightweight fission spectrometer 
provides a simple, reliable and precise method for 
determining neutron fluxes in a fixed neutron beam. 
The spectrometer, with a mass less than 120 g,
minimizes neutron scatter and energy degradation 
effects. It has a counting volume large enough to 
intercept the most energetic fission fragments, yet 
small enough to provide discrimination against 41Ar 
and fission-product decays. 
How it's done: 
The lightweight, low scatter unit is constructed of 
tinned steel, 0.025 cm thick, combined with Teflon 
insulators and a 0.15 cm brass or aluminum collec-
tion plate. A small can, 10 cm in diameter, provides 
the shell. The collection plate is located 3.5 cm from 
the fission foil. Ordinary miniature cable and rubber 
tubing are used for the gas inlet and outlet. 
The unit is normally operated in the ion chamber 
region using P-10 gas (90% Ar, 10% CH4) or pure Ar 
(continued overleaf) 
. 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=19680000174 2020-03-12T05:31:58+00:00Z
at a flow rate of approximately 50 cm 3sec-' and a 
collection plate voltage of + 1100 Vdc. 
Signals are fed through a linear preamplifier and 
amplifier into a multi- or single-channel analyzer or 
into a simple discriminator-scaler with equivalent 
results. The resolving time of the system is about 
1.5 tsec. With this resolving time and alpha and 
fission count rates up to 1400 sec- 1 , no alpha pileup 
is observed. However, if a single foil is used continu-
ously for many hours at higher count rates, a pileup 
due to 41 Ar and fission product decay is detectable. 
Even this pileup can be eliminated by flow rates in 
excess of 100 cm 3sec or a waiting time of a few 
hours. The fission foil, usually l00.tg cm- 2 or less, is 
mounted inside the can lid to avoid shadowing by the 
lid edges. 
Spectra from this fission spectrometer compare 
quite favorably with those obtained from more mas-
sive and complex gridded chambers or from solid-
state detectors.
Notes: 
I. Additional information is contained in Low Scat-
ter, Lightweight Fission Spectrometer, by N. A. 
Frigerio, The Review of Scientific Instruments, 
vol. 36, no. 7, p. 1048-1049, July 1965. 
2. Inquiries concerning this innovation may be di-
rected to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60.439 
Reference: B68-10174 
Source: N. A. Frigerio
Biological and Medical Research Division 
(ARG- 10094) 
Patent status: 
Inquiries about obtaining 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 68-10174
	 Category 02


Low scatter lightweight fission spectrometer constructed for biological research

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