Diapositiva 1

1
Design and test of an albedo personal neutron
dosemeter based on PADC detectors
R. Bedognia, A. Espositoa, G. Gualdrinib, R.
Mishrac, S. P. Tripathyc a INFNLNF Frascati
National Laboratory, via E.Fermi n. 40, 00044
Frascati, Italy b ENEA-Radiation Protection
Institute, Via dei Colli n. 16, 40136 Bologna,
Italy c BARC - Bhabha Atomic Research Centre,
Trombay, 400085 Mumbai, India
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Introduction
? Introduced in late 60s, the albedo dosimetry
technique relies on the detection of the neutrons
thermalized and back-scattered by the human
body. ? Due to the availability of high
sensitivity techniques for the measurement of
thermal neutrons, the detection of low energy
neutrons back-scattered from the body is in
principle easier than the direct detection of
fast neutrons. ? Together with etched-track
detectors (mainly PADC) and superheated
emulsions, albedo dosemeters are among the most
popular fast neutron personal dosemeters.
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Introduction
? Albedo dosemeters are typically based on pairs
of 6LiF / 7LiF based thermoluminescence
detectors. Advantages are high sensitivity to
thermal neutrons, reusability, simple read out,
low cost. ? Combined TLD-albedo (lt0.1
MeV) / track detectors (0.1 20 MeV) techniques
have been proposed to obtain reasonably flat
energy response. Main problems ? The high
photon sensitivity of TLDs causes large
uncertainties, especially in workplaces with
dominant photon component. ? Complex equipment
needed chemical lab, track reader, TLD reader.
? The response in terms of Hp(10) shows an
important energy dependence (it decreases with
the energy and falls dramatically above 0.1 MeV),
? The variability of the response with the
angle of incidence is usually very limited.
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Introduction
This work presents the study of an albedo
dosemeter based on a planar PADC covered by a 10B
screen. This will constitute a low-energy
section to be combined with a directly exposed
PADC (high-energy section) to form a wide energy
range personal dosemeter (photon insensitive,
only PADC equipment needed). The energy and
angle dependence of the response was investigated
through extensive simulations (MCNP 4C) and
irradiation at different reference fields
(241Am-Be, 252Cf and 252Cf(D2O)) and angles of
incidence (0, 20, 40, 60) in the
calibration laboratories of ENEA-Bologna and INFN
Frascati.
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Experimental set up
Detectors PADC (0.1 DOP) 2.5 cm x 3.5 cm x
0.14 cm from Intercast Europe (Parma,
Italy). Converter BE10 from Kodak (99 10B,
thickness 50 mm, density 1.45
g.cm-3) Etching 90 minutes in KOH with
Normality 6.25 N at 70C Read out semi
automatic reader developed at INFN-LNF, total
scanned area 1.92 cm2. Resolution 2.5 mm/pixel
All irradiations were perfomed in terms of
Hp(10,a) on ISO water filled phantom. Room- and
air-scattered radiation was taken into account
with ISO techniques (ISO 8529-2).
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Simulations
The nine dosemeters were individually modeled.
Simulations were performed for all neutron fields
and angles of incidence used in the
experiment. In addition, a series of
mono-energetic neutron beams plus thermal
neutrons were simulated to predict the energy
dependence of the response. The number of (n,a)
reactions in the converter was expected to be
proportional to the measured track density on
PADC. Total and phantom-scattered neutron
spectra were calculated at the entrance of the
boron converter.
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Total and phantom-scattered neutron spectra
Pure thermal
252Cf(D2O)
252Cf
241Am-Be
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Energy dependence of the fluence response
SF percentage of (n,a) reactions in the
converter due to neutrons back-scattered from the
phantom (estimated with a self flagging card)
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Energy dependence of the fluence response
r number of (n,a) reactions induced in the
converter per unit incident neutron fluence.
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Energy dependence of response in terms of Hp(10)
rH number of (n,a) reactions induced in the
converter per unit incident neutron fluence.
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Comparison experiment / simulations
Ratio simulation/experiment for all neutron
fields and angles of incidence
Standard deviation 5 Average value 71410
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Other dosimetric performances
Angle dependence of the response The average of
the responses a different angles differs by less
than 30 from normal incidence. Criterion ISO
21909 satisfied.
Detection limit DL, Dose level at which a known
fraction of false negatives may occur 5 in
this work) 40 mSv for Am-Be (conservative)
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Conclusions
? The response of a PADC based albedo personal
dosemeter was investigated with MCNP 4C and
experimental tests in reference neutron fields
with different angles of incidence. Good
agreement (within 5) was found between
calculations and measurements . ? As expected,
the energy dependence of the response is very
large from thermal to MeV neutrons. ? The angle
dependence of the response is very limited. ?
The achievable DL in workplace fields may be
lower than 40 mSv The studied configuration is a
good candidate as low energy section of a
combined dosemeter (albedo direct detection)
fully based on PADC and photon-insensitive