Development of neutron survey meter using prescila neutron probe
Nuclear Science and Technology, Vol.8, No. 4 (2018), pp. 26-33
Development of neutron survey meter
using prescila neutron probe
1,*
1
Luong Thi Hong , Nguyen Tien Phong , Pham Thi Bich , Nguyen Duc Huyen
1 Center for Non – Destructive Evaluation; 140 Nguyen Tuan, Thanh Xuan, Hanoi, Vietnam
(Received 20 December 2018, accepted 20 February 2019)
Abstract: This paper presents the design and validation of a neutron survey meter. The meter consists
of a PRESCILA neutron probe (with good sensitivity, directional response, gamma rejection, and
enhanced high-energy response to 20 MeV) and an electrometer developed at Non-Destructive
Evaluation center. The homogeneity response of the PRESCILA neutron probe was investigated as a
function of distances from the
source in order to obtain the appropriate distance for
accurate count-rate measurements using the neutron survey meter. A system consists of the
PRESCILA neutron probe and the Ludlum Model 2326 electrometer was then used for measuring
neutron ambient dose equivalent rates in the range from 50 cm to 200 cm with the step of 25 cm. The
relationship between the count-rates and neutron dose equivalent rates (in the distance ranged from 50
to 200 cm) were deduced to validate the proper operation of the neutron survey meter.
Keywords: Neutron survey meter, PRESCILA neutron probe, Ludlum electrometer.
MeV), high sensitivity, and moderate gamma
rejection [1].
I. INTRODUCTION
A
neutron survey meter normally
This paper presents the fabrication of a
neutron survey meter for the purpose of safety
assessment, which consists of a set of five
PRESCILA neutron probes and a self-
developed electrometer designed by Non-
Destructive Evaluation (NDE) center. The
validation of proper operation of the survey
meter was also investigated using the neutron
standard fields at the Institute for Nuclear
Science and Technology (INST) [2,3].
constitutes of an electrometer and a single or
multiple neutron detectors. Principle of a
conventional neutron survey meter is based on
1960s technology that relies on a large neutron
moderator assembly surrounding a thermal
detector to achieve a response function over a
limited energy range. Such neutron survey
meter reveals an ergonomic challenge, being
heavy and bulky, and have caused injuries
during radiation protection surveys [1].
Another defect of the traditional neutron
survey meter is a poor high-energy response
above 10 MeV, which makes them unsuitable
for neutron dose measurements at high-energy
neutron facilities (i.e. medical linear
accelerators). The PRESCILA Proton Recoil
Scintillator developed Los Alamos National
Laboratory is a light-weight (2 kg) probe with
extended high-energy response (up to 20
II. MATERIAL AND METHOD
A. PRESCILA neutron probe
The model 42-41L PRESCILA neutron
probe was developed by the Health, Safety, and
Radiation Protection Division at the Los Alamos
National Laboratory. The probe is the
combination of a dual-scintillator ZnS(Ag)+6LiF
and ZnS(Ag) + plastic, using both fast and
©2018 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute
LUONG THI HONG et al.
thermal neutron detectors, made it possible to
The 3D view of the PRESCILA probe is
shown in Fig.(1a) and the structure is shown in
Fig.(1b). Where, “11” is a light guide; “11a” is
the central penetration beamline; “12-13-14” are
the top, bottom, and the side plates with 5%
borated polyethylene, respectively; “12a” is a
central aperture; “15” is central apertures for
insertion of fast neutron scintillators
(ZnS(Ag)+plastic); “16” is a cadmium (Cd)
filter; “17” is a thermal neutron scintillator
(ZnS(Ag)+6LiF); “18” is a plastic spacer; 19 is
a photomultiplier tube (PMT) and “20” is a
handle [4].
balance the overall energy response and provide
adequate response in the crossover region
between the thermal and fast neutron energy.
The probe is capable of excellent
sensitivity (40 counts per minute per μSvh-1 for
241Am-Be source) and extended high-energy
response up to 20 MeV. The directional
response is uniform within 15% over a wide
range of energy. The linearity response has
been confirmed up to a dose rate of 20 mSvh-1.
The probe has high-dose gamma rejection up
to 2 mSv.h-1 [1].
(a): 3D outer view
(b): different components
Fig. 1: PRESCILA (Model 42-41L) neutron probe [4].
The fast neutrons are detected using a
powders on the back of a Lucite disc. The
thermal neutrons are detected via the
phosphor scintillator of ZnS(Ag) powder via
(
)
proton recoil process. The blue light
(n,α) reactions [5].
as the result of scintilation process is then
recorded by a photomultiplier tube. The
detection process can be expressed as
follows555 [5]:
B. Commercial neutron survey meter
A commercial gamma/neutron survey
meter including a model 2363 Ludlum
electrometer and the model 42-41L
PRESCILA neutron probe was used to measure
neutron dose equivalent rates at various
distances from the source. The measured
values were then used to compare with the data
measured using the developed neutron meter.
(
)[
]
(
)
(
)
Thermal and epithermal neutrons are
detected using a thermal scintillator (Eljen EJ-
420P) which is a mixture of F and ZnS(Ag)
27
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE
The commercial neutron survey meter
has an energy-compensated GM (allowing
gamma detection) and a neutron detector based
on proton recoil scintillation process. The
meter is significantly lighter than other
traditional neutron survey meter. Fig. 2 shows
the commercial neutron survey meter [1].
C. Electrometer designed by NDE
In this paper, an electrometer
developed by NDE is presented, which is
constituted of an analog electronic block of
different components such as high voltage
power supply, pre-amplifier, pulse shapper,
real time clock module, counter timer, etc.
The microprocessor-based central processing
unit controls the counting, calculating,
displaying of results on a graphic LCD
monitor, etc. The electrometer console is
user-friendly interface. Pulse discrimination
was accounted into the electronic block for
gamma rejection. This electrometer was then
connected to the model 42-41L PRESCILA
neutron probe to constitute a neutron survey
meter. The electronic diagram of the neutron
meter is shown in Fig. 3.
Fig. 2. Commercial neutron survey meter consists
of a model 2326 Ludlum electrometer and a
model 42-41L PRESCILA neutron probe [5].
Fig. 3. Neutron survey meter consists of a commercial model 42-41L PRESCILA neutron probe and an
electrometer designed at NDE (the component inside the dashed line).
28
LUONG THI HONG et al.
The design goal of neutron survey meter
being optimized in a wide energy range [1]. In
this section, the investigation of the effective
distance of the self-developed neutron survey
meter for measurements is performed.
Therefore, the count rates obtained by the self-
developed neutron survey meter exposed to 02
bare 241Am-Be neutron sources (80 mCi and 40
mCi) were recorded and analyzed. In this
experiment, two neutron sources were placed
at the distance range from 0 cm to 75 cm (with
a step of 15 cm) from the detector surface. The
experimental setup can be seen in Fig. 4. The
total count rates measured by the self-
developed neutron survey meter are tabled in
Table I (each value is averaged over 05
measurements).
is to balance the under response in the range
from about 0.1 MeV to 2 MeV by an over
response below 0.1 MeV that would give the
most accurate results for a range of practical
field spectra, with the applied radiation
weighting factor (WR =10), so when the
calibration of the dose rates is much simpler
(usually using the energy of the neutron
emitted by 241Am-Be or 252Cf). More detailed
characteristics of neutron probe used in the
neutron survey meter developed by NDE can
be found in the manufacturer’s specifications
[5]. A combination of lead and cadmium
filters is assembled with the scintillator probe.
The lead filter (2.84 cm in diameter, 0.038 cm
in thickness) is used to reduce the
scintillator’s sensitivity to low-energy
photons, while the cadmium filter (1.91 cm in
diameter, 0.039 cm in thickness) is used to
reduce the thermal neutron response below the
cadmium cut-off energy.
Model 42-41L
Electrometer
PRESCILA
neutron probe
designed by NDE
III. RESULT AND DISCUSSION
02 bare 241Am-Be
neutron sources (80
and 40 mCi)
A. Effective distance of the self-developed
neutron survey meter
The use of both fast and thermal
scintillator allows the energy response function
of the self-developed neutron survey meter
Fig. 4. Experimental setup for investigating the
effective distance.
Table I. The count rates (cpm) measured by the self-developed neutron survey meter at various distances
from the two neutron sources.
Electrometer
designed by NDE
Center
Side I
Side II
Side III
D
(cm)
Count
rate
Count
rate
Count
rate
Count
rate
Count
rate
A
SD
P (%)
(cpm)
(cpm)
(cpm)
(cpm)
(cpm)
0
1106.2
342.8
188.2
1512.0
399.0
197.0
1447.4
394.8
202.6
1473.6
413.8
209.6
1616.0
416.8
217.0
1431.0
393.4
202.9
180.9
31.5
14.9
60.0
70.0
90.0
15
30
29
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE
45
60
75
117.8
83.8
61.8
131.2
87.6
64.0
121.8
86.0
62.4
130.4
88.8
63.8
135.8
88.8
62.8
127.4
87.0
63.0
14.5
6.6
95.0
99.0
99.5
5.4
-
P is the detection probability which is a function of distance from the source (probability as
Confidence intervals of value average with each distance);
D is distance from source to detector;
-
-
-
-
-
Side I, II, III, IV is four sides with fast scintillators;
Center is the side with thermal scintillator;
SD is standard deviation;
A is the count rates averaged over all of five sides.
The effective distance for the
laboratory at INST. The count rate measured
by the self-developed neutron survey meter in
each of five sides (4 thermal sides and one
central fast side) of the neutron probe were
performed. The measurements were performed
at three distances of 50cm, 75cm and 100cm.
The data were collected for X-Y planes and
shown in 360º of Z plane (angle from 0º to
360º with a step of 30º). The results are
presented in Fig. 5 (0o at the top position). The
measured values show the maximum deviation
of about 14% over 360o responses.
measurements using the self-developed neutron
survey meter was chosen at the distances that
the “P” values greater than 95%. From Table 1,
it implies that the measurements should be
conducted at a distance more than 45 cm from
the source.
B. Angular dependence of the self-developed
neutron survey meter
The angular dependence of the self-
developed neutron survey meter was
investigated at the secondary standard dosimetry
1201
629
386
1203
541
1512
573
348
346
329
377
517
1184
554
1139
SDD100
Fig. 5. Homogeneity response of the self-developed neutron survey meter at various angles.
30
LUONG THI HONG et al.
In order to verify the accuracy of the
C. Validation of the self-developed neutron
survey meter
self-developed neutron survey meter, the meter
was calibrated at the secondary standard
dosimetry laboratory at the INST. The total
neutron ambient dose equivalent rates (unit in
µSv.h-1) were measured at distances from 60 to
150 cm in the standard neutron field of a bare
241Am-Be source. The response due to the
direct component of neutron field was then
analyzed and compared with the reference
values reported in Refs. [2, 3] to confirm the
proper operation of the self-developed neutron
survey meter. The results are shown in Table 2.
Once the effective distance from the
source to the detector was obtained for the self-
developed neutron survey meter, the neutron
ambient dose equivalent rates were measured
using the commercial neutron survey meter in
the distance range from 50 cm to 150 cm. The
total count rates were also measured using the
self-developed neutron survey meter in the
same distance range from 50 cm to 150 cm
with the step of 25 cm. The total count rates
were then fitted as a function of the neutron
dose equivalent rates (measured using the
commercial neutron survey meter). The
relationship is shown in Fig. 6 and used to
compute the displayed function of the self-
developed neutron survey meter (i.e. display
unit of the self-developed neutron survey meter
is functioned in µSv.h-1).
From Table II, ones can see that the
neutron ambient dose equivalent rates
measured by the NDE center self-developed
neutron survey meter and those in the free
field are in good agreement within 1% . This
means that the self-developed neutron survey
meter can be well applied in the practical
neutron ambient dose equivalent rate
measurements.
Fig. 6. Neutron ambient dose equivalent rates (μSv/h in Y axis, measured by a commercial neutron survey meter)
as a function of the total count rates (cpm in X axis, measured by the self-developed neutron survey meter).
31
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE
Table II. The neutron ambient dose equivalent rates (due to the direct component of a neutron standard field
of a bare 241Am-Be source) measured by the self- developed neutron survey meter in the comparison
with the standard ones.
Ambient dose equivalent rates
Standard ambient dose
Distance
(cm)
measured by the self- developed
neutron survey meter
(μSv/h)
Deviation
(%)
equivalent rates
(μSv/h)
60
70
402
295
226
145
101
64
400
294
225
144
100
64
0.49
0.37
0.43
0.68
0.98
0.00
80
100
120
150
V. CONCLUSIONS
ACKNOWLEDGEMENT
In this work, the neutron survey meter
was introduced based on the commercial
neutron probe (model 42-41L PRESCILA)
and a self-developed electrometer by NDE.
The effective distance for the measurements
using the meter was investigated. The other
characteristics of the neutron survey meter
were also evaluated, such as the
homogeneity response, the dose equivalent
rate response. Moreover, the neutron survey
meter was also calibrated at the secondary
standard dosimetry laboratory at the
This work has been financially
supported by the Vietnam Ministry of Science
and
Technology
under
the
project
The
ĐTCB.02/17/TTNDE
(2017-2018).
authors would like to thank Dr. Le Ngoc
Thiem, MSc. Nguyen Ngoc Quynh and Ms.
Dang Thi My Linh at INST for their valuable
comments during the manuscript preparation.
REFERENCES
[1]. Richard H. Olsher, David T. Seagraves,
Shawna L. Eisele, Christopher W. Bjork,
William A. Martinez, Leonard L. Romero,
Michael W. Mallett, Michael A. Duran,†and
Institute
for
Nuclear
Science
and
Charles R. Hurlbut; “Prescila:
A
new
Technology in order to confirm the proper
operation. In the future, other characteristics
of the neutron meter (dose range sensitivity,
energy range response, gamma rejection,
etc) will be verified to reconfirm the
practical funtionality in neutron safety
assessment.
lightweight neutron rem meter”; Health
Physics, Volume 86, Number 6, June 2004.
[2]. Le Ngoc Thiem et al.; “Characterization of a
Neutron Calibration Field with 241Am − Be
Source using Bonner Sphere Spectrometer”;
68-74, 2018.
32
LUONG THI HONG et al.
[3]. Le Ngoc Thiem et al.; “Neutron Calibration
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Neutron
Survey
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Ludlum
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74. Ann. ICRP 26 (3-4), 1996.
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