Diapositiva 1

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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
• Politecnico di Milano, Dipartimento di Energia,
  Piazza Leonardo da Vinci, 32, Milano, Italy
• 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.

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A CR39 SSNTD coupled with a Boron converter was
used as thermal neutron detector. The detector
exploits the (n,a)reactions on the 10B inside the
boron converter. Both a particles and 7Li ions
are produced in the reaction, and they are
detected by the CR39 SSNTD.
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• a 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, 98C,
  40 minutes

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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,a reaction on 10B) and others
coming from other sources, such as NORM, dust
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With this noise reduction methods, a background
track density of 33 tracks/cm2 has been
achieved. The sensitivity to thermal neutrons
have been measured in 6E-3 tracks/n
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• A passive REM counter designed to host at its
  centre a CR39BE10 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.

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The detector has been tested in high energy
fields both at GSI Cave A and at CERF

• 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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• The 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.
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Measurements at CERFCERN-EU reference field
The high energy neutrons are obtained by 150 Gev
protons impinging on metal targets (Cu-Al).
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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)
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The measurements show a good agreement between
the passive REM counter and other extended range
REM counters (such as the CERN Active Linus)
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• 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 708 tracks,
  that means a 12 uncertainty.This is
  unavoidable
• 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.

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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.
• It has an uncertainty significantly higher than
  active REM counters due to poisson uncertainty.
• 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)