Fast neutron spectrometer combines helium filled proportional counters with solid-state detectors to achieve the properties of high efficiency, good resolution, rapid response, and effective gamma ray rejection

Moler, R. B.

Preston, C. C.

English

NASA Technical Reports Server

November 1966	 Brief 66-10555 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
A Fast-Neutron Spectrometer of Advanced Design 
The problem: 
To design and develop a fast-neutron spectrometer 
for spectral measurements requiring a combination of 
good resolution, high efficiency, and rapid response. 
Accurate measurements are required of fast-neutron 
fluxes of 108/cm 2/sec to 10 10/cm 2/sec, within a 
gamma field as high as 1.5 x 106R/hour and a time 
limit of 5 seconds. 
The solution: 
An instrument that combines He3-filled propor-
tional counters with solid-state detectors to achieve 
the properties of high efficiency, good resolution, 
rapid response, and effective gamma-ray rejection. 
How it's done: 
Using the He3(n,p)T reaction as a neutron con-
verter, if the total energy of the secondary particles, the 
proton and triton, is absorbed, the recorded spectrum 
is easily interpreted. Since this reaction has a Q-value 
of 0.760 Mev, the energy of a recorded neutron is 
well above most background (interference), and a 
resolution of 30 to 50 Ke v can be achieved. Neutrons 
whose absolute energies are 2 to 3 times this value 
(as low as 100 Ke y ), produce peaks that can be 're-
solved from those peaks produced by thermal neu-
trons. A number of advantages are realized if a 
method that makes use of the volume of He 3 gas 
present in the solid-state sandwich spectrometer is 
developed. The He 3 gas is used as a proportional 
counter. Resolution is improved by adding the energy 
deposited in the proportional counter to that absorbed 
in the solid-state charged-particle detectors. Coin-
cidence between the solid-state detector and the pro-
portional counter results in greatly reduced back-
ground.
An extension of the above use of He 3 gas as a pro-
portional counter, is to divide the volume into two 
proportional counters. This arrangement permits 
the use of particle identification techniques to elimi-
nate gamma interactions, He3 recoils, deuterons from 
the He3(n,d)D reaction, alpha particles from the 
Si28(n,a)Mg25 reaction, and many of the protons from 
the Si 28(n,p)Al28 reaction. 
A neutron spectrometer has been developed to in-
corporate the above considerations. The He 3 volume 
is divided into two proportional counters separated by 
a series of wires that defines the electric field. Coin-
cidence between the two proportional counters is 
achieved, and a proton is identified through its 
(dE/dX) xE product. 
Note: 
Inquiries concerning this invention may be directed 
to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B66-10555 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546. 
Source: Christopher C. Preston
and Robert B. Moler
of ItT Research Institute 
under contract to

Marshall Space Flight Center
(M-FS- 1664) 
Category 01 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person 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 such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19660000553 2020-03-11T17:44:39+00:00Z


A fast-neutron spectrometer of advanced design

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