Development of neutron survey meter using prescila neutron probe

er normally 
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 
MeV), high sensitivity, and moderate gamma 
rejection [1]. 
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]. 
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)+
6
LiF 
and ZnS(Ag) + plastic, using both fast and 
LUONG THI HONG et al. 
27 
thermal neutron detectors, made it possible to 
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 
241
Am-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]. 
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]. 
(a): 3D outer view (b): different components 
Fig. 1: PRESCILA (Model 42-41L) neutron probe [4]. 
The fast neutrons are detected using a 
phosphor scintillator of ZnS(Ag) powder via 
proton recoil process. The blue light ( ) 
as the result of scintilation process is then 
recorded by a photomultiplier tube. The 
detection process can be expressed as 
follows555 [5]: 
( )[ ] ( )
 ( ) 
Thermal and epithermal neutrons are 
detected using a thermal scintillator (Eljen EJ-
420P) which is a mixture of 
 F and ZnS(Ag) 
powders on the back of a Lucite disc. The 
thermal neutrons are detected via the 
 (n,α) 
 reactions [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. 
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE 
28 
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]. 
Fig. 2. Commercial neutron survey meter consists 
of a model 2326 Ludlum electrometer and a 
model 42-41L PRESCILA neutron probe [5]. 
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. 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). 
LUONG THI HONG et al. 
29 
The design goal of neutron survey meter 
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 
241
Am-Be or 
252
Cf). 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. 
III. RESULT AND DISCUSSION 
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 
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 
241
Am-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). 
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. 
D 
(cm) 
Center Side I Side II Side III Side IV 
A SD P (%) Count 
 rate 
 (cpm) 
Count 
 rate 
(cpm) 
Count 
 rate 
(cpm) 
Count 
rate 
(cpm) 
Count 
rate 
(cpm) 
0 1106.2 1512.0 1447.4 1473.6 1616.0 1431.0 180.9 60.0 
15 342.8 399.0 394.8 413.8 416.8 393.4 31.5 70.0 
30 188.2 197.0 202.6 209.6 217.0 202.9 14.9 90.0 
Electrometer 
designed by NDE 
Model 42-41L 
PRESCILA 
neutron probe 
02 bare 
241
Am-Be 
neutron sources (80 
and 40 mCi) 
Electrometer 
designed by NDE 
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE 
30 
The effective distance for the 
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 
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 (0
o
 at the top position). The 
measured values show the maximum deviation 
of about 14% over 360
o 
responses. 
Fig. 5. Homogeneity response of the self-developed neutron survey meter at various angles. 
386 
346 
377 329 
348 
629 
573 
517 554 
541 
1201 
1512 
1184 1139 
1203 
SDD100
45 117.8 131.2 121.8 130.4 135.8 127.4 14.5 95.0 
60 83.8 87.6 86.0 88.8 88.8 87.0 6.6 99.0 
75 61.8 64.0 62.4 63.8 62.8 63.0 5.4 99.5 
- 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. 
LUONG THI HONG et al. 
31 
C. Validation of the self-developed neutron 
survey meter 
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
). 
In order to verify the accuracy of the 
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 
241
Am-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. 
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). 
DEVELOPMENT OF NEUTRON SURVEY METER USING PRESCILA NEUTRON PROBE 
32 
Table II. The neutron ambient dose equivalent rates (due to the direct component of a neutron standard field 
of a bare 
241
Am-Be source) measured by the self- developed neutron survey meter in the comparison 
with the standard ones. 
Distance 
(cm) 
Standard ambient dose 
equivalent rates 
(μSv/h) 
Ambient dose equivalent rates 
 measured by the self- developed 
neutron survey meter 
(μSv/h) 
Deviation 
 (%) 
60 402 400 0.49 
70 295 294 0.37 
80 226 225 0.43 
100 145 144 0.68 
120 101 100 0.98 
150 64 64 0.00 
V. CONCLUSIONS 
 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 
Institute for Nuclear Science and 
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. 
ACKNOWLEDGEMENT 
This work has been financially 
supported by the Vietnam Ministry of Science 
and Technology under the project 
ĐTCB.02/17/TTNDE (2017-2018). The 
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. 
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LUONG THI HONG et al. 
33 
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