Energy Response Characteristics of Several Neutron Measuring ...

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Energy Response Characteristics of Several Neutron Measuring Devices
Determined By Using the Scattered Neutron Calibration Fields of KAERI
B.H. Kim 1 , J.L. Kim 1 , S.Y. Chang 1 , J.K. Chang 1 , G. Cho 2
1 Korea Atomic Energy Research Institute, P.O.Box105, Taejon, 305-600, Korea
2 Department of Nuclear Engineering,
Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea
INTRODUCTION
Neutron measuring devices used for radiation monitoring in the field of radiation protection have very
different response characteristics with energy. Mismatches of measured quantities with the variation of energy
spectra come from monitoring instruments that are not calibrated using neutron fields similar to the workplaces.
This is the reason why the Realistic Neutron Calibration Fields (RNCF) (1) should be used in the calibration of
neutron remmeters and dosimeters if possible.
Normal calibrations with the radioactive neutron sources recommended by the International
Organization for Standardization (ISO) are performed using the direct component of incident neutron, mainly
fast neutrons, to the devices to be calibrated at the scatter-free or low scatter facility. The effects of scattered
neutron in the reference point were corrected or subtracted in determining the response of the neutron measuring
devices and generally the dose contribution of these scattered neutrons to the total dose is not significant. Most
neutron working fields to be monitored have a considerable component of scattered neutrons with relatively low
energy including thermal neutron and multi-directional or not unidirectional. Typical neutron remmeters overrespond
in the intermediate energy region and the albedo type personnel dosimeters do so in the low energy
region. Therefore, when the calibration is performed, it is necessary to use the representative neutron field
encountered in the workplace for minimizing an error due to the spectral difference at the time of monitoring.
At recently, the construction and use of RNCF were suggested by ISO to calibrate the neutron
measuring devices, because the RNCF would be assumed as closer to the real working field and more
appropriate than the neutron reference fields of ISO-8529 (2).
Ten kinds of the scattered neutron fields for calibration have been constructed using a means similar
to the new draft of the ISO (PTB’s method) at the Korea Atomic Energy Research Institute (KAERI) and used to
determine the energy response characteristics of several neutron measurement devices used popularly for
radiation protection purposes.
MATERIALS AND METHOD
Irradiation Facility and Neutron Fields of KAERI
The neutron irradiation room of KAERI is a bunker room whose dimensions are 8 m long, 6 m wide
and 6 m high, and it is enclosed with 60 cm thick concrete walls and ceiling. Two kinds of neutron sources, 252 Cf
and 241 AmBe can be used for calibration. 252 Cf is placed at the center of the room and moved from storage to an
irradiation position of a height of 2.9 m at the time of irradiation and this source is mainly used for routine
calibration. In using 252 Cf, bare and D 2 O moderated neutron spectra are available. D 2 O moderator assembly is a
0.53 mm thick Cd-covered sphere with diameter of 32.3 cm to cut-off thermal neutrons, so there are few thermal
components in the reference calibration spectrum.
Descriptions of the scattered neutron fields of KAERI constructed in the irradiation room are shown in
Table 1. Two kinds of ordinary phantoms, a Poly-Methyl Meta-Acrylate (PMMA) and the ISO water filled
phantom, used for irradiation of personal dosemeter and the shadow cone were placed to produce the scattered
neutron fields between the source and the calibration point. The shadow cone was made of iron with a length of
50 cm. The front end was 20 cm iron with a diameter of 30 cm and the back end 30 cm air with a diameter of 35
cm. All reference positions were fixed at a center-to-center distance of 100 cm between the source and the
detectors except for place E, where it is behind the wall in the irradiation room. The thickness of the concrete
wall is 21 cm. The reference calibration point in this study was usually 100 cm from the effective center of the
source.
The dosimetric quantities including the neutron fluence rate at the test point were determined by
means of the Bonner Multi-sphere Spectrometry System (BMSS) (3). The calibration of the BMSS was
performed in the KAERI neutron irradiation room by using the 252 Cf source with the same technique as done by
Liu et al. (4). Integral properties such as the spectral mean neutron energy, Eave., the neutron flux and dose rate
on the reference date of Jan. 1, 2000, the fractional contribution of thermal neutrons to the total fluence, ϕ th /ϕ,
and the fluence to ambient/personal dose equivalent conversion factors averaged over the neutron energy spectra,
h*(10) and h p (10), with its averaged mean neutron energy, E*ave. and Ep,ave. are summarized in Table 2 for ten
scattered neutron fields (5). The mean neutron energies and the conversion factors for the ambient and personal
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dose equivalents were calculated using the values obtained from the interpolation method of cubic spline for the
conversion factors for mono-energetic neutrons given by ICRP 74 (6).
Table 1. Description of KAERI scattered neutron fields.
Notation
Description
252 Cf
(bare)
252 Cf
(D 2 O)
[A]
[B]
[C]
[D]
[E]
[F]
[G]
[H]
[I]
Direct and scattered; unmoderated source only
Scattered; using the PMMA phantom (40 x 40 x 15 cm 3 , contacted to the source guide holder)
Scattered; using the shadow cone (distance from the source to the front end : 32 cm)
Scattered; using the PMMA phantom(same as B) and polyethylene sheet with 5% boron
(61x 61 x 5 cm 3 , distance from the source to the surface : 40 cm)
Behind the concrete wall in the irradiation room (distance from the source : 385 cm)
Direct and scattered; moderated source only
Scattered; using the PMMA phantom(same as B, distance from the source : 32 cm)
Scattered; using the shadow cone (distance from the source to the front end : 32 cm)
Scattered; using the water filled phantom (30 x 30 x 15 cm 3 , 32 cm from the source)
241 AmBe [J] Direct and scattered; unmoderated source only
Table 2. Integral properties of several scattered neutron fields in the neutron irradiation room of KAERI.
Field
Eave. a)
(MeV)
Flux
(n.cm -2 sec -1 )
ϕ th /ϕ
(%)
Dose rate b)
(mSvhr -1 )
h*(10)/ E*ave c)
(pSvcm 2 /MeV)
h p (10)/Ep,ave c)
(pSvcm 2 /MeV)
[A] 1.282 3019 10.2 3.557 327/1.63 339/1.64
[B] 0.444 1449 51.2 0.673 129/1.42 133/1.42
[C] 0.532 1260 24.8 0.940 207/1.03 214/1.03
[D] 0.692 761 36.4 0.442 161/1.76 167/1.78
[E] 0.282 323 53.1 0.109 93/1.20 96/1.22
[F] 0.461 2734 9.4 1.095 111/1.46 115/1.47
[G] 0.205 1009 48.6 0.261 69/1.18 72/1.19
[H] 0.171 1060 38.9 0.348 91/0.70 94/0.70
[I] 0.184 1028 44.9 0.279 75/0.95 78/0.98
[J] 3.436 75 6.4 0.092 347/3.29 362/3.30
a)
spectral mean energy.
b) ambient dose equivalent rate, H*(10).
c)
fluence to ambient (and personal) dose equivalent conversion factor and dose equivalent averaged mean
energy reffered to the conversion factors given by ICRP-74 (6).
Detector Response
Response characteristics of several neutron measurement devices were determined by getting the
quotients of the detector indication by the dose equivalents given in table 2, which were determined by using the
BMSS of KAERI. All neutron detectors were calibrated in the field of D 2 O moderated 252 Cf source and the
indication values of the detector multiplied by the calibration factor. At the time of calibration, the effect of
scattered neutrons for each neutron detector was corrected by the semi-empirical fitting method of ISO (7). The
six kinds of neutron detectors and their calibration factors are listed in Table 3.
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Table 3. Neutron detectors and calibration factors for D 2 O moderated 252 Cf.
Detector model
Detection method
Calibration
facor 1)
NG-2 (NRC, USA) Cylindrical moderator and cylindrical BF 3 proportional counter 0.987
ESP2/NRD (Eberline, USA) Spherical moderator and cylindrical BF 3 proportional counter 1.029
Ludlum 12-4 (Ludlum, USA) Spherical moderator and spherical BF 3 proportional counter 0.917
REM-500 (HPI, USA) Tissue equivalent proportional counter 0.968
Dineutron (Nardeux, France) Two spherical 3 H proportional counter (2.5 & 4.2 inch) 0.794
A300 (Teledyne, USA) Thermoluminescence dosimeter 0.979 2)
1) Calibration factor for the D 2 O moderated 252 Cf.
2) Dosimeters for calibration were irradiated on the ISO water-filled phantom at a distance of 50 cm using the same
neutron source as 1) (8).
RESULTS AND DISCUSSION
Indications of all remmeters were lower than the quantities determined by BMSS , as a reference. This
means that BMSS overestimates the dose equivalents for most neutron fields to be monitored and this trend of
BMSS is still reasonable in view of conservative radiation protection. When the calibration was performed in the
field of D 2 O moderated 252 Cf, dose equivalents were over estimated from 20 % to 80 % roughly for the
moderator type remmmeters in these measurements. These values can be reduced by the adoption of a calibration
factor for a 252 Cf source. This is not common because the discrepancies between the reading values and the
conventional true dose equivalents basically result from the big change in detector response with energy. Two
ways to solve this problem were suggested by Naismith et al. (9): one is to use a similar neutron field to the
workplace in the calibration of neutron detectors for use and the other is to apply the correction factors which are
categorized for the specific neutron fields using the relations between the detector responses and some databased
neutron fields.
The responses and calibration factors were obtained for the ten kinds of neutron fields of KAERI and
are given in Tables 4 and 5, and Figures 1 and 2. Even though these fields are not representative for the
workplaces, the figures given in Tables 4 and 5 show that it is necessary to correct the response according to the
neutron fields. In the case of TLD used in the field of the 252 Cf source only, [A], corrections more than four times
should be applied to the readings if TLDs are conventionally calibrated in the field of D 2 O moderated 252 Cf. All
moderator type remmeters have the similar response shape with energy and the calibration factors ranged from
1.22 ~ 1.92, whereas REM-500 and Dineutron show different responses in the case of more scattered neutron
fields or at low energy region. REM-500 has relatively low responses at low energies as shown in the paper by
Thomas (10). In the case of Dineutron, special care is necessary to correct the low sensitivity when it is used in
more scattered neutron fields for monitoring.
Although it is not possible to determine the energy response by the definition without the use of
mono-energetic neutron sources, this paper shows what type of instruments are necessary to a specific neutron
field and how important it is to correct the response of neutron detectors used in workplace monitoring.
Table 4. Responses of neutron detectors in several scattered neutron fields.
Field
[A] [B] [C] [D] [E] [F] [G] [H] [I] [J]
Detector
NG-2 0.748 0.676 0.617 0.591 0.520 0.705 0.602 0.552 0.595 0.822
ESP2/NRD 0.690 0.666 0.609 0.594 0.571 0.741 0.620 0.586 0.666 0.745
Ludlunm 0.621 0.623 0.606 0.578 0.546 0.696537 0.646 0.648 0.687 0.616
REM-500 0.605 0.584 0.557 0.526 0.463 0.466 0.354 0.348 0.362 0.786
Dineutron 0.663 0.217 0.648 0.393 0.206 0.641 0.299 0.473 0.346 0.477
A300(TLD) 0.189 0.968 - 1.885 0.679 0.828 1.256 - - -
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Table 5. Calibration factors of neutron detectors in the several scattered neutron fields.
Field
Detector
[A] [B] [C] [D] [E] [F] [G] [H] [I] [J]
NG-2 1.337 1.480 1.620 1.693 1.921 1.419 1.660 1.813 1.681 1.217
+- 0.012 0.006 0.009 0.008 0.031 0.007 0.014 0.016 0.014 0.020
ESP2/NRD 1.449 1.501 1.642 1.684 1.752 1.350 1.612 1.705 1.502 1.342
+- 0.019 0.047 0.051 0.051 0.063 0.024 0.103 0.057 0.105 0.083
Ludlum 1.611 1.605 1.651 1.732 1.832 1.436 1.548 1.543 1.456 1.622
+- 0.007 0.026 0.027 0.048 0.099 0.015 0.073 0.044 0.049 0.121
REM-500 1.652 1.712 1.795 1.900 2.159 2.146 2.823 2.877 2.765 1.272
+- 0.108 0.212 0.185 0.221 0.714 0.192 0.848 0.454 0.314 0.210
Dineutron 1.508 4.619 1.544 2.542 4.856 1.560 3.349 2.114 2.887 2.098
+- 0.004 0.079 0.010 0.059 0.078 0.010 0.019 0.053 0.000 0.017
A300(TLD) 5.270 1.034 - 0.531 1.473 1.208 0.796 - - -
+- 0.843 0.051 - 0.025 0.076 0.034 0.032 - - -
Fig. 1. Responses (up) and calibration factors (down) of remmeters for the scattered neutron fields.
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Fig. 2. Responses (left) and calibration factors (right) of TLD for the scattered neutron fields.
ACKNOWLEDGEMENTS
This study was the partial product of the national projects for long term nuclear energy development
supported by the Ministry of Science and Technology.
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