Discriminator in nuclear particle detection system distinguishes nuclear particle type and energy among many different nuclear particles. Discriminator incorporates passive, linear circuit elements so that it will operate over a wide dynamic range

Lockwood, J. A.

St. Onge, R.

English

NASA Technical Reports Server

• 
September 1969	 Brief 69-10481 
NASA TECH BRIEF U 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
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from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Improved Pulse Shape Discriminator for Fast Neutron-Gamma Ray

Detection System 
Photomultiplier
Integrator 
- - - -	
( /Ldt) Pulse 
	
_	 I	 I 
I	 I' 
	
Dynode	 E	 ___ 
Fast Pulse 
Transformer 
	
Anode	
 
Pulse 
_ L	
VI - 	 I 
/ I 
Pulse Shape Discriminator 
Fast Neutron-Gamma Ray Detection System 
A simple, inexpensive pulse shape discriminator 
has been designed for use in a nuclear particle detec-
tion system (including a photomultiplier and two-
dimensional pulse height analyzer) to enable the 
system to simultaneously distinguish nuclear particle 
type and energy among many different nuclear 
particles (or radiation), e.g., y, e, n, p, jt, and ce. Pre-
vious systems can only discriminate between a 
maximum of two particles. The discriminator incor-
porates only passive, linear circuit elements, so that
it will operate reliably over a wide dynamic range. 
Other known pulse shape discriminators employ 
diodes and/or transistors. Since the circuit is 
composed of only passive, linear elements, it has an 
inherently high temperature stability. Additionally, 
the system is independent of photomultiplier high-
voltage and gain changes since the discriminator 
operates from only one photomultiplier output. 
Other pulse shape discriminators generally require 
two or three separate multiplier outputs, which make 
(continued overleal) 
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a 
the detection systems sensitive to any changes in 
photomultiplier characteristics and necessitate com-
plicated readjustments. 
As with other nuclear particle detection systems, 
the photomultiplier used with the new discriminator 
receives light pulses from an appropriate scintillator 
which absorbs energy from the nuclear particles and 
radiates (scintillates) this absorbed energy in a 
particular combination of modes. The time constants 
for the decay or relaxation of scintillation pulses 
depend upon the type and energy content of the ex-
citing particle. Discrimination among the different 
types of particles is possible because the amplitude 
ratio of fast to slow components of the relaxations 
differ for each type of particle. The new system 
compares the slower secondary decay amplitudes to 
the total integrated amplitudes of all decays. The 
reason for using this method of comparison lies in 
the saturating, nonlinear characteristics of photo-
multipliers; that is, very fast, high-intensity decays 
cannot be faithfully followed by the photomultiplier 
because of space charge limitations. 
Operation of the pulse shape discriminator is based 
on the partially destructive and partially constructive 
interference of a pair of electronic pulses derived 
from one scintillator-photomultiplier pulse. This one 
original pulse may be either from a photomultiplier 
dynode or anode (as illustrated). This pair of pulses 
is obtained from one pulse by means of a commer-
cially available fast pulse transformer. Such a trans-
former will produce two conjugate, 180 0 out-of-phase, 
unipolar pulses from one input pulse, which are ex-
actly in time coincidence. These iwo opposite-polarity 
pulses are then separately operated upon electroni-
cally. One pulse is RC-differentiated with a short time 
constant, thus producing a bipolar pulse. The other, 
an opposite-polarity pulse, is merely recombined by 
addition to suitably interfere and thereby cancel the 
first part of the RC-differentiated bipolar pulse. This 
cancellation produces an extremely shape-sensitive
output without the use of any nonlinear devices. The 
output is finally integrated before entering an ampli-
fier. Since this amplified unipolar pulse is related to a 
differentiation, it is conveniently defined as the 
(dL/dt) pulse and is particularly dependent on particle 
type. 
Simultaneously from a dynode, as illustrated, a 
pulse is taken, integrated for several decay periods, 
and then amplified. This amplified pulse is thus de-
fined as the f Ldt pulse and is related to particle 
energy. The two pulses (dL/dt) and f Ldt are plotted 
against each other on a two-dimensional field (multi-
parameter pulse-height analyzer) to show the particle 
separation and relative energy content. 
Notes: 
1. A disadvantage of this device is that it requires 
a two-dimensional pulse height analyzer, whereas 
most other pulse shape discriminator systems use 
only a one-dimensional analyzer. 
2. It has been ascertained that some organic liquids 
and alkali halides may be used as scintillators for 
this system. It is believed that the system may also 
be used with solid state nuclear particle detectors. 
3. No further documentation is available. Inquiries 
may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: B69-10481 
Patent status: 
No patent action is contemplated by NASA. 
Source: R. St. Onge and J. A. Lockwood of 
University of New Hampshire 
under contract to 
NASA Headquarters 
(I-I QN -10151) 
Brief 69-10481	 Category Of


Improved pulse shape discriminator for fast neutron-gamma ray detection system

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