A fast coincidence circuit with pulse height selection


Material Information

A fast coincidence circuit with pulse height selection
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
4 p. : ill. ; 27 cm.
Bell, P. R
DeBenedetti, S
Francis, J.E., Jr
Clinton National Laboratory
U.S. Atomic Energy Commission
Atomic Energy Commission
Place of Publication:
Oak Ridge, Tenn
Publication Date:


Subjects / Keywords:
Coincidence circuits   ( lcsh )
Electronic circuits   ( lcsh )
Nuclear counters   ( lcsh )
Nuclear physics -- Instruments   ( lcsh )
federal government publication   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )


"Date Declassified: April 22, 1947"
Statement of Responsibility:
by P.R. Bell, S. DeBenedetti, J.E. Francis, Jr.
General Note:
Manhattan District Declassified Code

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005024644
oclc - 288524548
System ID:

This item is only available as the following downloads:

Full Text
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P R. Bell
8. DeBenedetti
1. E. Francis, Jr.

Clinton National Laboratory

Date Declasulfied: April 22, 1947

This document is for official use.
Its issuance does not constitute authority
for declassification of classified copies
of the same or similar content and title
and by the same authors.

Technical Information Branch, Oak Ridge, Tennessee
AEC, Oak Ridge, Tenn., 5-2-49--750-A4938

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By P. R. Bell, S. DeBenedetti, and J. E. Francis, Jr.


The output signal of a linear amplifier may have a rise time of 1-2 x 10-7 seconds. Therefore,
the output of a pulse height selector operating on this signal has a variation of delay from the
original signal of about this amount. Since this delay depends on the pulse height, the pulse height
selector output is unsuitable for fast coincidence work. In the instrument to be described this dif-
ficulty is largely removed by feeding the amplifier signal into two separate channels: the first in
a pulse height selector which produces a rectangular pulse (gate) longer than the amplifier signal
when this signal height falls between two adjustable limits of voltage; the second channel limits, dif-
ferentiates and delays the amplifier signal so that it falls within the "gate" duration. The pulses
from the two channels are fed to a coincidence circuit whose output reproduces the constantly delayed
and sharpened amplifier pulse only when the gate pulse is present. This scheme has been employed
to obtain simultaneous or delayed coincidences between pulses (with a height falling between two
specified limits) from one chamber, and pulses greater than a known height from a second counter.
The resolving time is approximately .4 microseconds.

S *

When coincidence counting is to be done with signals whose amplitude varies from pulse to
pulse some difficulties are found that are not found when dealing with signals of uniform size.
Signals of varying amplitude might arise from ionization chambers or proportional counters and
it is generally desired to select from coincidence purposes only those signals whose amplitude lies
above a selected value or those whose amplitude lies between two selected values.
The output signal or "gate" from a circuit pulse height selector that selects those pulses
greater than some set amplitude cannot begin until the signal voltage has reached this triggering
level. As a consequence the gate will begin at various times after the signal starts from almost
no delay at all for a very large pulse to a delay equal to the time required for the signal to rise
to the very crest for a signal that barely triggers the pulse height sele tor.
If the output of the pulse height selector is used for coincidence work the resolving time can-
not be less than the rise time of the signal plus the length of time required for the operation of the
coincidence circuit.
One method for reducing the effect of this variable delay is to use the output of the pulse
height selector only to select the desired pulses between which coincidence is to be determined
and perform the coincidences between the original signals after they have been suitably limited
and delayed. Figure 1 shows one arrangement. Here the signals from counter or chamber 1 are
amplified and fed into a pulse height selector and into delay line No. 1. The output of the delay line
is sharply limited so that a signal not much above the level of the background noise produces full

Presented orally at a meeting of the American Physical Society in Washington, D. C., on
May 1, 1947.
MDDC 799 1

MDDC 799

MDDC 799

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output. This limited signal is differentiated by an RC circuit and the resulting pulse fed to a coin-
cidence circuit together with the gate from the pulse height selector. The gate from the pulse
height selector is made equal to or slightly larger than the length of the differentiated pulse plus
the maximum variation in delay due to the variation in signal height. The delay line delay is long
enough so that the differentiated pulse will always fall inside the gate time. The output of the
coincidence circuit will then be a sharp pulse with a substantially fixed delay from the beginning
of the signal and this pulse will be present only when the signal was high enough to trigger the
pulse height selector.
The signal from counter or chamber 2 is amplified and fed to two pulse height selectors. The
two pulse height selectors are adjusted to define a region of pulse height. The lower limit pulse
height selector will trigger soonest and its gate must be delayed so that if the upper pulse height
selector is triggered the resulting slightly longer gate pulse will overlap the first. These two
gates are fed to an anti-coincidence circuit so that a pulse will come out of this anti-coincidence
circuit when the lower pulse height selector is triggered but no pulse (or an opposite sign pulse)
will come out if both pulse height selectors are triggered. This is the usual differential pulse
height selector.
The signal from amplifier 2 is also fed into a delay line and the delayed signal is limited and
differentiated as was the case with signal 1 and the resulting pulse is fed into coincidence circuit
No. 2 with the anti-coincidence gate. The output of coincidence circuit No. 2 is then a sharpened
pulse with a fixed delay from its signal and is present only when the signal lies between the levels
of the upper and lower pulse height selectors.
The output pulses from coincidence circuit No. 1 and coincidence circuit No. 2 can be fed into
coincidence circuit No. 3 and if delay lines No. 1 and No. 2 have the same delay the pulses will be
in coincidence if the original signals were.
An auxiliary delay line D4 may be inserted between coincidence circuit No. 1 or coincidence
circuit No. 2 and the third coincidence circuit to allow the random coincidence rate to be deter-
mined. An alternate method is to make delay line No. 1 shorter than delay line No. 2 thus al-
lowing the gate pulse from coincidence circuit No. 1 to be shorter. The difference in delays in
channel 1 and 2 is then made up by an auxiliary delay line which can be removed to measure the
random coincidences.
By means of an instrument of this pattern using amplifiers of about 0.2 gp ec rise time an
overall resolving time of about 0.2 to 0.25 p sees was obtained.
Figure 2 shows the block diagram of another arrangement that differs from the first in that
the two signals are sharpened and fed into a fast coincidence circuit at once and the desired pulses
are sorted from all those obtained by the operation of the various pulse height selectors. The
advantage of this circuit is that only a few operations are performed on the signals until coincidence
is obtained. After this no damage can be done to the resolving .time by the delay lines and other
coincidence circuits in the system. In this arrangement the output gate from pulse height selector
No. 1 is put into coincidence circuit No. 2 with the anti-coincidence gate from channel No. 2; coin-
cidence is then sought by circuit No. 3 between the gate from coincidence circuit No. 2 and the out-
put pulse from the sigr 1- coincidence circuit No. 1. It is necessary to delay this pulse to allow it
to fall into the time occupied by the gate from coincidence circuit No. 2.


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