Transcript Diapositiva 1 - Istituto Nazionale di Fisica Nucleare

24th ICNTS-Bologna 2008
A passive REM counter based on
CR39 SSNTD coupled with a
boron converter
Agosteo, S.1 Caresana, M.1 Ferrarini. M 1 Silari.M2
1) Politecnico di Milano, Dipartimento di Energia, Piazza Leonardo da
Vinci, 32, Milano, Italy
2) CERN, 1211 Geneva, 23 CH
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REM counters are neutron
dosemeters made of a thermal
neutron detector surrounded by
a shell of moderating
materials, such as polythene,
with metal insets.
They are designed to have a
spectral response that is
proportional to the fluence to
ambient dose equivalent H*(10)
conversion coefficients.
The counts of the thermal
neutron detector are
proportional to H*(10)
REM counters designed for high
energy applications usually have
heavy metal insets (Lead,
Tugnsten) to extend their
response to high energy
neutrons up to 1 -2 GeV.
A CR39 SSNTD coupled with a Boron converter was used as
thermal neutron detector.
The detector exploits the (n,α)reactions on the 10B inside the
boron converter. Both α particles and 7Li ions are produced in
the reaction, and they are detected by the CR39 SSNTD.
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α particles with energies
up to 1.47 MeV are
produced. Their range in
the material is in the
order of 7 μm. This
makes etching a very
delicate procedure.
The etching time has
been chosen as a
compromise between
track radius and contrast.
The etching is made with
NaOH 25% solution,
98°C, 40 minutes
Tracks are
almost perfectly
round and they
have similar
parameters
(such as
greyscale,
sharpness)
It is so possible to plot the parameters and to distinguish
between tracks coming from thermal neutrons (via the n,α
reaction on 10B) and others coming from other sources, such as
NORM, dust
With this noise reduction methods, a background track density
of 3±3 tracks/cm2 has been achieved. The sensitivity to thermal
neutrons have been measured in 6E-3 tracks/n
A passive REM counter
designed to host at its
centre a CR39+BE10
detector has been
designed.
The detector has been
calibrated at Politecnico di
Milano, with a Pu-Be
source.
Using an enriched boron
converter its sensitivity is
7 tracks/cm2mSv.
The background, due to
the background reduction
algorythm used, is 3 ±3
tracks/cm2 .
This implies a LDL of
2 mSv.
The detector has been tested in high
energy fields both at GSI Cave A and at
CERF
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At GSI high energy neutrons are generated by
400 MeV/u C ions impinging on a carbon
target.
Measurements have been made inside the ernty
maze and out of the cave shielding, with dose
rates ranging between 2-40 mSv/h.
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measurements have been carried out in the frame of the
CONRAD project, involving several european laboratories.
The measurement in OC13 has been made with an integral
dose of 12 μSv, and is still expoloitable. And has a statistical
significance.
Measurements at CERF
CERN-EU reference field
The high energy
neutrons are
obtained by 150
Gev protons
impinging on
metal targets (CuAl).
Several
measurements
have been made,
intercomparing
with different
instruments.
(PoliMi-CERN)
An intercomparison was made between the passive REM
counter, the CERN acive Linus , and two commercial
units (Thermo-electron Wendi, and Berthold)
The measurements show a good agreement between the passive
REM counter and other extended range REM counters (such as
the CERN Active Linus)
The detector has a high uncertainty (if compared to active
REM counters), because of
 Poisson uncertainty: 7 tracks/cm2μSv mean that a 10
μSv measurement is made with 70±8 tracks, that
means a 12% uncertainty.
This is unavoidable
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Lot uncertainty: the tracks are formed at the very
surface of the detector. Different lot of detectors may
have slightly different properties in the first microns
from the surface, that can cause a systematic shift of
the mesurements (up to 20%). This can be avoided
calibrating every lot of CR39.
The Boron converter may be contaminated (~0.1 mBq/cm2) with
NORM. This can cause problems in long term measurements. This
problem can be solved because the tracks can be distinguished in
the greyscale to radius plot.
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The method is very sensitive, and it can provide reliable
measurements even with integral doses down to 10 μSv.
The LDL is in the order of 2 μSv.
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It has an uncertainty significantly higher than active
REM counters due to poisson uncertainty.
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It is especially suitable for routine area environmental
monitoring where a large number of measurements
point are needed (es. large plants), or where a large
active environmental neutron monitoring system is not
justified (es. conventianal radiotherapy centres)