Li6 sandwich-type neutron spectrometer is equipped with proportional counter for particle identification. System uses current-sensitive preamplifiers to minimize pile-up of gamma-ray and particle pulses

Baker, S. I.

Moler, R. B.

Zagotta, W. E.

English

NASA Technical Reports Server

June 1973	 B73-10116 
NASA TECH BRIEF 
Marshall Space Flight Center 
NASA Tech Briefs announce now technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Fast-Neutron Spectrometer Developments 
Preamplifier	 Amplifier 
Triton Detector 
Prean,plifer	 Amplifier 
Attencator 
-	 Proportional Counter 
"N
LiF Layer 
Alptra 
Detector 
Preamplifier	 Amplifier
Discriminator  
Arsal,thtr	 Generator 
- 
Unit 
Discriminator M	 Gate
 Gate 
Amplifier 
Passiw	
- Multichannel 
Analyzer 
Capacitor
And
Atteouato,
8lock Diagram of L i ii Spectrometer and
Particle Identification System 
Neutron spectrometers have been used for monitoring 
and studying nuclear reactors. Tests have shown that, 
when high-energy neutrons are studied, reactions in the 
silicon detectors interfere with the measurements. 
This problem may be overcome to a significant extent 
by a Li 6 sandwich-type neutron spectrometer equipped 
with a proportional counter for particle identification. 
The unique feature of this system is its ability to 
minimize the pile-up of gamma-ray and particle pulses 
by using current-sensitive preamplifiers. 
The figure shows a block diagram of the spectro-
meter and particle identification system. The spectro-
meter utilizes two 900 ohm-cm silicon surface barrier 
detectors, one an uncoated triton detector and the other 
an alpha detector that is evaporation coated with 
500 pg/cm 2 of U 6 F. The spectrometer shield is made of 
8% gadolinium oxide and 80% boron (enriched to 92%
B'° to enhance absorption and allow a smaller shield 
thickness). 
Background gamma radiation is rejected by combining 
the characteristics of the detector reaction [Li 6 (n,t) H4] 
with the use of nanosecond electronics. The Li 6 (n,t) H4 
reaction has a high Q (4.785 MeV) and yields coinci-
dental alpha and triton particles. The current-sensitive 
preamplifiers form signal pulses of approximately 15 
nanoseconds. The probability of including random 
gamma-ray events is reduced through the use of 15-
nanosecond wide electronic gates that are opened only 
by coincident alpha particle and triton events. In the 
absence of background radiation, peak height analysis of 
thermal neutron events give a peak that is approximately 
300 keV full-width at half-maximum and remains less 
than 500 keV until the gamma-ray field strength 
reaches around 20,000 R/hr.
(continued overleaf) 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
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As are most semiconductor detectors, this one is 
susceptible to radiation damage. The primary limitation 
is the total fast neutron dose which the detector can 
tolerate. However, a considerable dosage is required 
before resolution begins to fall off. This is because the 
effect of preamplifier noise on resolution is much greater 
than the intrinsic resolution of the unirradiated de. 
tectors. One of the detectors is able to withstand a total 
fission neutron dose of 4 x 1012 neutrons per cm 2
 and 
the other 1013 before failure. 
The electronics in this system have been miniaturized. 
A blocking circuit may be added to eliminate the pile-up 
of good coincidence events in the slow (microsecond) 
portion of the circuitry, and a dead time meter could be 
used at high flux levels. The electronic circuitry, without 
the multichannel analyzer, occupies a volume of less 
than 820 cm  and requires only 7 watts from a 28-volt 
direct current source.
Note: 
Requests for further information may be directed to 
Technology Utilization Officer 
Marshall Space Flight Center 
Code A&PS-TU 
Marshall Space Flight Center, Alabama 35812 
Reference: B73-10116 
Patent status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
Patent Counsel 
Marshall Space Flight Center 
Code A&PS-PAT 
Marshall Space Flight Center, Alabama 35812 
Source: R. B. Moler, W. E. Zagotta, and

S. I. Baker of

lIT Research Institute

under contract to

Marshall Space Flight Center 
(MFS.22279)
. 
. 
. 
B73-10116
	 Category 03


Fast-neutron spectrometer developments

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