Transcript Diapositiva 1 - Istituto Nazionale di Fisica Nucleare

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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
INFN–LNF 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.
● 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.
● Combined TLD-albedo (<0.1 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.
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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(D O)) and angles of incidence (0°, ±20°,
2
±40°, ±60°) in the calibration laboratories of ENEABologna and INFN Frascati.
Experimental set up
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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%
density 1.45 g.cm-3)
Etching:
90 minutes in KOH with Normality 6.25 N at 70°C
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.
Roomand
air-scattered
radiation was taken into
account with ISO techniques
(ISO 8529-2).
10B,
thickness 50 mm,
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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.
Total and phantom-scattered neutron spectra
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Pure thermal
252Cf
252Cf(D
241Am-Be
2O)
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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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r=
Energy dependence of the fluence response
number of (n,a) reactions induced in the converter per
unit incident neutron fluence.
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rH =
Energy dependence of response in terms of Hp(10)
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 = 714±10
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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