=Paper=
{{Paper
|id=Vol-2023/176-181-paper-27
|storemode=property
|title=Neutron generators and DAQ systems for tagged neutron technology
|pdfUrl=https://ceur-ws.org/Vol-2023/176-181-paper-27.pdf
|volume=Vol-2023
|authors=Evgeniy Bogolyubov,Alexander Gavryuchenkov,Maxim Karetnikov,Dmitri Yurkov,Valentine Ryzhkov,Vladimir Zverev
}}
==Neutron generators and DAQ systems for tagged neutron technology==
https://ceur-ws.org/Vol-2023/176-181-paper-27.pdf
Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
NEUTRON GENERATORS AND DAQ SYSTEMS FOR
TAGGED NEUTRON TECHNOLOGY
E.P. Bogolyubov, A.V. Gavryuchenkov, M.D. Karetnikov, D.I. Yurkov,
V.I. Ryzhkov, V.I. Zverev
Dukhov Research Institute of Automatics (VNIIA), 22, ul. Sushchevskaya, Moscow 127055, Russia
E-mail: bogolubov@vniia.ru
At the T(d,n)He4 reaction each 14 MeV neutron is accompanied by a 3.5 MeV alpha- particle emitted
in the opposite direction. A position- and time-sensitive alpha-detector measures time and coordinates
of the associated alpha particle which allows determining time and direction (tags) of neutron escape.
The tagged neutron technology is based on a time and spatial selection of events that occur when a
tagged neutron moves through the object. The VNIIA ING-27 neutron generators produce intensive
beams of tagged neutrons in a wide cone angle with the high spatial and time resolution of tagged
neutrons provided by the pixelated fast alpha-detector. Requirements to a DAQ system for various
tagged neutron devices are reported. The architecture and parameters of the DAQ system based on
preliminary online selection of signals by analog front-end electronics and transmission of only useful
events for subsequent computer processing are considered. The examples of tagged neutron devices
for various applications are discussed.
Keywords: neutron generators, DAQ, alpha-detector, gamma-detector, tagged neutron technology
© 2017 Evgeniy P. Bogolyubov, Alexander V. Gavryuchenkov, Maxim D. Karetnikov, Dmitri I. Yurkov,
Valentine I. Ryzhkov, Vladimir I. Zverev
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
1. Introduction
Neutron analysis is in great demand in many areas of science and industry. It is due to unique
neutron properties such as high penetrability because of zero electric charge and selective interaction
with nuclei of chemical elements regardless of their chemical or aggregate state.
Neutron analysis is based on the measurement of secondary gamma-neutron radiation
generated as a response to initial neutron radiation. One of the main problems here is a high
background caused mostly by gamma rays generated while interactions of neutrons with the
components of the device or surrounding objects, by products of the decay of generated radionuclides,
etc. In recent years, the Nanosecond Tagged Neutron Technology (NTNT) has been rapidly
progressing. Owing to spatial and time selection of events the NTNT allows the level of background
radiation to be substantially reduced [1].
Figure 1 displays a block diagram of a typical NTNT device. Initial energies and directions of
a neutron and an alpha-particle from T(d, n)4He reaction are unambiguously interrelated. A position-
sensitive (multipixel) alpha-detector produces a time-stamp Tα and coordinates of the recorded alpha-
particle, thus providing its motion direction and the exit time by correction for the alpha-particle
velocity. Using this data one can evaluate the neutron exit time, vector of motion, and energy in the
direction towards an interrogated object, i.e. the neutron can be “tagged” by the associated alpha-
particle recorded. This technology is also known as Associated Particle Imaging (API) due to a
significance of the associated alpha-particle for tagging the neutron.
While inelastic scattering of fast neutrons by nuclei, the energy spectrum of the resultant
gamma rays has pronounced signatures, i.e. it is unique for different chemical elements, and can be
used for their identification. A gamma-detector measures the energy and time Tγ of arrival of the
gamma-quantum. A distance L from the target of the neutron generator to the point of emission of a
gamma-quantum generated while inelastic scattering of the fast tagged neutron can be determined by
means of measuring the time interval Δt between time stamps of detection of the gamma-quantum and
alpha particle associated with the neutron, Δt= Tα- Tγ. Knowing the distance L and vector of neutron
motion, one can calculate the location of the nucleus emitted the gamma-quantum.
Figure 1. Block diagram of NTNT device
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
For analysis of bulk inhomogeneous objects, the interrogated object can be conditionally
divided by separate elementary volumes (voxels). A position of each voxel can be defined in
spherical-like coordinates represented by coordinates of the activated pixel (giving the vector of
escape of the tagged neutron) and the time interval Δt between detection of the gamma-quantum and
alpha particle. A chemical composition within the voxel is assumed to be uniform. A size of the voxel
is determined by the spatial and time resolution of the tagged neutron device. It can be as low as 5-8
cm along the tagged neutron beam and 2-3 cm in transverse direction. An aggregate of voxels gives a
3D map of chemical composition of the interrogated object.
The possibilities of time and spatial selection of events stipulates following advantages of
NTNT compared to other methods of neutron analysis:
- 3D imaging of interrogated object;
- High effect/background ratio.
NTNT is a very convenient tool for study of fast neutrons interaction with different nuclei. It
was used for measuring angle correlations of products of inelastic scattering of fast neutrons in
numerous works. Currently, a cycle of experiments for study (n, n′γ) and (n, 2n′) reactions is run at
theTANGRA setup (Figure 4) in the Joint Institute for Nuclear Research (JINR), Dubna, Russia [2].
The results of experiments can be essential for nuclear physics and astrophysics.
The NTNT capability to measure gamma-spectrum at any specific point of the interrogated
volume and high penetrability of fast neutrons and gamma-rays make it possible to locate and identify
suspicious items in bulk objects or identify items shielded by massive walls or even thick water layer.
In mine industry, it allows detecting large (the expected sensitivity is close to 5 karat) diamonds in
kimberlite ore before the crushing stage [3]. It simply detects excess carbon at a particular point of the
kimberlite sample. The carbon spectrum is characterized by 4.44 MeV and first escape 3.93 MeV
peaks. These lines are rather weak in the spectrum of pure kimberlite.
The most promising applications of the NTNT are certainly connected with the remote
detection of explosives [1]. Practically all explosives feature higher (more than several times) nitrogen
concentration compared to harmless nitrogenated materials such as wool, polyurethane, and nylon.
Moreover, ratios of concentration of carbon, nitrogen, and oxygen nuclei in high explosives are within
rather narrow range. The NTNT capability to detect key chemical elements C, N, and O increases
probability of explosives detection compared to technologies based on only nitrogen determination
[4]. In addition, when a substance is identified based on several elements, it is possible to normalize
their concentration (for example calculate C/(C+N+O), N/(C+N+O), O/(C+N+O) ratios), which is
very important in case of strong neutron and gamma-ray attenuation in bulk objects.
2. Generators of tagged neutrons
A key component of NTNT is a neutron generator with a built-in position sensitive alpha-
detector. Currently, off-the-shelf generators of tagged neutrons are manufactured by Thermo
Scientific, USA (API-120 series [5]), Adelphi Technology Inc., USA (DT108API) [6], and Dukhov
Research Institute of Automatics (VNIIA), Russia (ING-27 series) [7]. Peak intensity of API-120 is as
high as 2·107 1/s. Peak intensity of DT108API and ING-27 is close to 1·108 1/s. The scintillating
alpha-detectors are used in API-120 and DT108API neutron generators. Typically, the scintillator is
coupled through an optical plate with a position-sensitive PMT, and the crystal geometry depends on
the PMT input window geometry. However, there are only few models of position-sensitive PMT that
can be used for this task. Hamamatsu H8500 and H9500 have the largest size of input window (52x52
mm) among them. Assembling of several position-sensitive PMT at the input flange of the neutron
generator to increase the sensitive area of alpha-detector heavily complicates the problem.
VNIIA produces specialized silicon and GaAs semiconductor-type detectors for NTNT
neutron generators (ING-27). They have certain technological and economical advantages over
scintillating alpha-detectors, providing at the same time similar basic operating parameters (radiation
hardness, time resolution, detection efficiency). It makes possible to assemble any reasonable
configuration (Figure 2) of the alpha-detector limited only by a geometry of a neutron generator
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
flange. A tagged neutron beam profile that defines NTNT angular resolution depends on the pixel size,
diameter of deuteron beam spot on the target, and distance between the target and the pixel. Limiting
angular resolution is determined by chromatic and geometry ion-optic aberrations, effects of alpha-
particle scattering in the tritium target and neutron scattering in the target holder; and in case of ING-
27 it is close to 0.04 – 0.05 rad.
Figure 2. Configurations of semiconductor alpha-detectors to be built-in ING-27 neutron generator: a)
3x3 pixels, 10x10 mm pixel size, b) 8x8 pixels, 6x6 mm, c) 1x15 pixels, 6x6 mm, d) 16x16 pixels, 2x2
mm; e) 8x8 pixels, 10x10 mm, f) 12x16 pixels, 6x6 mm; g) 16x16 pixels, 4x4 mm
The signal from the alpha-detector pixel is used as a time stamp for measuring the distance L
from the target of the neutron generator to the point of emission of a gamma-quantum (by the time
interval Δt ). An accuracy (time resolution) is determined by the signal front slope, its fluctuation, and
signal/noise ratio. These parameters are deteriorated with the pixel area, and a time resolution of the
alpha detector varies from 0.7 ns (Figs 2a- 2d configurations) to 2,5-3 ns (Figs 2e- 2g configurations).
3. DAQ systems for tagged neutron devices
Taking into account a diversity of types of interrogated objects (from mail parcels and mobile
phones to cargo containers and vehicles) and applications of NTNT, different NTNT systems can be
suggested. They can be roughly divided into several types:
- Mobile systems for detecting mines, for inspection of unattended and suspicious objects, and
etc. The sounding by tagged neutrons and measurement of induced gamma-rays are carried out from
one side (Figure 3a). The mobile systems include 1 to 4 gamma-detectors. Over hundred of these
systems based on 9-pixel ING-27 neutron generator are currently used in the Moscow Metro for
control of suspicious objects [8].
- Stationary systems for explosives detection in a baggage and a hand luggage. The
interrogated object is located between the neutron generator and gamma-detectors, which quantity
varies from 6 to 12 (Figure 3b).
- Stationary systems for prospecting of diamonds in kimberlite ore, control of cargo containers
and vehicles. Gamma-detectors are aside the tagged neutron beam axis (Figure 3c). A total number of
gamma-detectors is several dozen pieces. Efficiency can be improved by means of usage of several
neutron generators and/or a movable neutron generator that is set in motion along a rail by a step
motor.
The counting rate of events for the first and second types of NTNT devices does not exceed
several thousands 1/s. There a number of relevant DAQ systems developed in various laboratories that
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
can provide the efficiency, energy and time resolution required for practical applications [8]. For the
third type, the total counting rate of alpha- detector can be more than 5∙106 1/s, the counting rate of
each gamma-detector is above 105 1/s, and the total rate of events can exceed (1-2)∙105 1/s .The
processing of such data stream without loss and distortion of information is a challenge to a DAQ
system.
a) b) c)
Figure 4. Types of NTNT systems: a) Mobile system, b) Stationary system for baggage and hand
luggage control, c) Stationary systems for control of cargo containers and vehicles. IO – interrogated
object, SH – shielding, NG – neutron generator, GD – gamma-detectors.
At present, there are two approaches to the problem of processing signals from NTNT devices:
(1) signal processing by analog front-end electronics, preliminary online hardware selection of signals
by several criteria (time, amplitude, overlapping) and transmission of only useful events to a PC, and
(2) complete digitization of signals from all detectors, and transmission of the data stream to an
intermediate computer for subsequent processing.
The NTNT devices implementing digitization of signals from all the detectors have a sampling
period as low as 7 − 10 ns. Very short signals from the detectors should be extended in time up to
several dozen ns to provide significant number of samples for time and amplitude measurements. It
may limit maximum signal rate.
For the analog front-end electronics, the main selection criterion is the presence of signals
from alpha- and gamma-detectors within the preset time window and amplitude ranges in the absence
of overlapped events. These principles were assumed as a basis for the NTNT data acquisition system
developed in VNIIA. The basic features of this system are as follows.
1. Timing is executed by a constant fraction discriminators (CFD) which parameters (fraction,
delay) are adjusted for the specific parameters of signals. The time interval Δt between recording the
gamma-quantum and alpha-particle is measured by a time-digital converter (TDC).
2. The TDC is initiated by the gamma-detector signal, though in a real life an alpha-particle is
recorded prior to the gamma-quantum. For that the signal of alpha-particle is transmitted through a
delay line and used as a STOP signal for TDC. It sufficiently reduces a triggering rate of TDC.
3. The useful events (alpha-gamma coincidences) are represented by 8 bytes containing the
codes of amplitude of gamma-detector signal, Δt time interval, number of the actuated pixel of the
alpha-detector, and number of the actuated gamma-detector. They also include a code of time from the
start of operation for study of dynamic objects. The events are transmitted to the computer as a
buffered data. The spectra of gamma-signals accumulated simultaneously regardless of coincidences
is also measured. It allows calibrate the gamma-detectors on-line by specific peaks of the gamma-
spectra.
4. The system is based on modular approach; the modules are bonded through a backbone bus.
The typical configuration includes one 32-channel module for alpha-detector and the required number
of 6-channel gamma-detector modules.
5. All channels of gamma- detector modules are independent and can record signals
simultaneously.
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
The time for processing of each event does not exceed 2 µs, the maximum rate of events
transmission is not less than 200 000 1/s for each gamma-detector module. A real required rate of
events is much less for any NTNT system. The achieved time resolution is as low as 1.0 − 1.2 ns for
fast LYSO and LaBr3 detectors, and 1.5 – 3.0 ns for relatively slow NaI(Tl) and BGO gamma-
detectors .
To diminish impact of overlapping of signals from alpha-detector, the time window for
recording the alpha-gamma coincidences can be changed by size and can be shifted by means of
hardware and software with the accuracy of 1 ns. Thus the time window can be adjusted to the
position of interrogated object along the tagged neutron beam. This is especially important for the task
of diamonds detection in kimberlite ore when the interrogated zone is restricted by a sampling tray. A
reduction of the time window decreases background.
4. Conclusion
The detecting equipment developed for the tagged-neutron technology ensures precise and stable
measurements of the event parameters at any reasonable recording rate of events and number of
gamma-detectors. It meets the requirements for the practical realization of the tagged-neutron
technology for various applications, including explosives detection in a baggage and a hand luggage,
control of cargo containers and vehicles, prospecting of diamonds in kimberlite ore.
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