Digital signal processing equipment for neutron coincidence
counting
J. Bagi, J. Huszti, K. Szirmai
Department of Radiation Safety
Institute of Isotopes of the Hungarian Academy of Sciences
Konkoly-Thege M. 29-33, 1121 Budapest, Hungary
E-mail: huszti@iki.kfki.hu
Abstract:
IKI has developed four kinds of digital processing units for neutron coincidence counting. These
consist of an FPGA based external unit connected via USB to a PC. One unit is essentially a virtual
shift register giving the multiplicity distribution for both gates in real time.
The Pulse Train Recorder is a list mode system for recording time intervals between consecutive
detector pulses. Measured data are transferred via USB line to the PC. The data acquisition program
saves data in a binary file and at the same time displays time interval distribution on the screen.
Calculating of total and coincidence count rates is performed post measuring by a fast evaluation
software. There is also a program for calculating Rossi-alfa distribution and dye away time. There is a
multichannel version of the Pulse Train Reader hardware with 16 parallel inputs. It eliminates impulse
losses due to merging pulses of different preamplifiers.
Hardware and software for replaying pulse trains recorded with the list mode system has also been
developed. Replaying a pulse train simulates a real source therefore it is called virtual source. It is
useful for training purposes as a wide variety of virtual sources can be used anywhere without the
need of any real radioactive sources on site.
Keywords: neutron coincidence counting, list mode, virtual source
1. Introduction
Neutron coincidence counting has been well established in recent decades. The method was
developed for determining effective Pu-240 content of plutonium containing samples and it has been
regularly used in safeguards measurements. It is based on the assumption that spontaneous fission
rate is proportional to Pu-240 mass. For this first order coincidence rate called Doubles must be
measured. This is made by conventional shift registers. In some cases simple multiplicity counting is
not sufficient. Multiplicity counting involves also the next order moment called Triples in the
calculation. Under certain assumptions the spontaneous fission rate and also induced fission and (α,n)
reactions rate in the sample can be calculated.
List mode is relatively new way of neutron coincidence measurements made possible by rapid
development of computational hardware. List mode devices give each incoming pulse a time stamp,
and the sequence of time stamps is written to a mass storage device. Recorded pulse trains are
usually evaluated after measurement. List mode devices have the advantage that the same pulse train
can be analysed with different parameters or even with different codes. Another benefit of saving raw
data, that in problematic cases all statistical information can be obtained from raw data, such as
follow-up distribution, coincidence rates, Rossi-α distribution and dieaway time.
In the Institute of Isotopes (IKI) a series of digital processing equipment for neutron coincidence
counting has been developed in the past years.
2. Virtual multiplicity spectrometer
Virtual instruments have their controls on the screen of a PC, operation intensive parts of the function
are, however, implemented in hardware.
The multiplicity spectrometer acts like a conventional shift register. It simulates the entire shifting chain
with predelay, R+A window, long delay and A window. For each incoming neutron multiplicity
spectrum items corresponding to the population number of R+A and A windows are incremented.
Resulting spectra are transferred through RS232 to the PC for calculating coincidence rates. Spectra
and rates can be seen while data acquisition on the screen.
Coincidence rates needed for solving point model equations are calculated as
n
S
 R  Ai
i 0
Tmeas
n

n
D
  A
i 0
i
Tmeas
n
 i R  A   i  A
i 1
i
i 1
i
Tmeas
n
 i  A
i
n
 n

i i  1


R  Ai   Ai   i1
i
R

A

i  Ai 


i

2
S  Tmeas  i1
i 1

T  i 2
Tmeas
where S, D and T denotes Singles, Doubles and Triples rate respectively. R+A and A are measured
multiplicity values for the two windows and Tmeas is measuring time.
n

3. Pulse Train Recorder PTR-02
Pulse Train Recorder is a list mode device. It accepts standard TTL pulses coming from a neutron
detector and measures time intervals between consecutive pulses. Collected data is sent through a
USB line to a PC and stored in a binary file. Data files can be evaluated after measurement.
PTR-02 works with follow-up times instead time stamps. Time intervals between successive neutron
pulses are measured as number of clock periods passed. Therefore follow-up times are integer
numbers. Resulting data files contain the same information as with time stamping but require less
space and can be processed faster.
Digitizing cause impulse loss if there are more incoming impulses in a clock period. Derandomizing
circuits compensated for this by a fast intermediate storage which delayed excess impulse processing
until the next free clock period. High clock rate of PTR-02 make this unnecessary.
Incoming neutron impulses have a statistical fluctuation but data transfer takes place in more or less
uniform distributed data packets. For this reason a large buffer with appropriate control logic is
needed. In order to avoid buffer overflow at high count rates, data are compressed before buffering.
This multiplies effective buffer capacity up to three times at high count rates.
Data acquisition is controlled by a program running under Windows. It displays continuously follow-up
distribution while data acquisition. The interactive graph is expandable or collapsible and this helps to
identify possible measuring problems or interesting features in the data. Data files have a header
block containing automatically filled-in and user entered information. The program can read back
previously recorded binary data files and graphs.
3.1 Multichannel version
Measurements must be often made at relative high count rates. Impulse loss in the detector due to
merging impulse trains from different preamplifiers is sometimes unacceptable high. In order to reduce
this effect merging of impulse trains is shifted to the Pulse Train Recorder. Thank to inherent data
buffering impulse trains can be merged loss free.
Multichannel hardware performed so well, that it is now used also for one channel measurements with
all but one channels disabled.
3.2 Technical data
Min. follow-up time: 15 ns
Time resolution: 10 ns
Maximal count rate: 3 MHz
Output: 30 ns TTL pulse on every detected input pulse
Data output: USB 2.0 port, max. 10 MB/s
High voltage: optional
4. Evaluation software
Coincidence values and Rossi-α distribution are calculated by two separate programs, because latter
requires much more time and is not always needed.
Data files consist of a header block and binary data. The header block contains three kinds of data:
structure identifiers, auto filled-in fields and user entered description fields. Binary data are
consecutive follow-up times in four-byte integer format.
4.1 Coincidence rate calculation
Coincidence rates are calculated by the formulas given at the virtual multiplicity spectrometer.
Calculations are made in a fractional part of the acquisition time. Processing time never exceeds a few
percent of acquisition time even at high count rates. The Neutron program took part in the Neutron
Coincidence Benchmark Test and outraged with its speed.
Predelay, gate width and long delay can be set prior to calculation. The program can handle virtual
unlimited high multiplicity numbers. The same data set can be evaluated with different parameters.
Distribution values and calculation results can be saved in a text file.
4.2 Calculation of Rossi-α distribution
Rossi-α distribution describes the detection probability of another neutron after a trigger event in
function of time. Random events have in this approach a uniform distribution whereas fission neutrons
are time correlated that is usually described by a single exponential term. Calculating this distribution
consumes much processor time even with integer arithmetic, that is why it is done by a separate
program.
t
Dieaway calculation is made by fitting N (t )  A  R  e  . The calculated distribution is 1024 s long
with 100 ns time bins. Distribution values and calculation results can be saved in a text file.
5. Virtual source
Neutron coincidence counting is one of the most important tools of nuclear safeguards. It requires
good knowledge and practical experience which require not only time but different kind of neutron
sources and detectors, too. There are only a few laboratories possessing all this. High efficiency
detectors can be hardly moved because of their large mass and transporting radioactive sources
especially nuclear ones involves a lot of administration.
With a virtual source neither a source nor a detector is needed for neutron coincidence training. A
virtual source replaces source and detector by a library of previously recorded impulse trains. It
reproduces previously recorded impulse trains just like they would come from a real detector. In this
way impulse trains recorded in different laboratories and with different detectors can be reproduced.
The virtual source VS-02 is essentially a replayer for list mode files recorded with the Pulse Train
Recorder hardware. With a generator software impulse trains with arbitrary impulse and coincidence
rates can be made for replaying.
Recorded data are read back from the file, and transferred to the VS-02 unit in compressed form. The
hardware stores incoming data into a FIFO buffer and processes them one by one. The
decompressed value is used to set the follow-up time of the generated output pulse.
A replayed PTR-02 impulse train reproduces original follow-up times with an average error as low as 5
ns. The resulting impulse train holds all coincidence statistical properties of the original one, so not
only the coincidence values S, D and T can be calculated but also Rossi-α distribution and dieaway
time.
Virtual source VS-02 is based on the same FPGA platform as Pulse Train Recorder.
5.1 Technical data
Maximal rate for periodic impulses: 3 MHz
Output: 30 ns TTL impulse
Time resolution: 10 ns
Data input: USB 2.0 port, max. 10 MB/s
Power supply: 5 VDC, 2.5 A
Compatible with IAEA equipment
6. Summary
Institute of Isotopes developed on a low cost hardware platform a family of digital signal processing
equipment for neutron coincidence counting.
The results show that the Pulse Train Recorder is a valuable tool for neutron multiplicity counting.
Data evaluation software for the list mode data files outraged in the ESARDA benchmark test with its
speed.
The Virtual Source unit for training and education purposes is unique.