A passive REM counter based on CR39 SSNTD coupled with a boron converterAgosteo, 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

24th ICNTS-Bologna 2008

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.

αα particles with particles with energies up to 1.47 energies up to 1.47 MeV are produced. MeV are produced. Their range in the Their range in the material is in the material is in the order of 7 order of 7 μμm. This m. This makes etching a very makes etching a very delicate procedure.delicate procedure.

The etching time has The etching time has been chosen as a been chosen as a compromise between compromise between track radius and track radius and contrast.contrast.

The etching is made The etching is made with NaOH 25% with NaOH 25% solution, 98°C, 40 solution, 98°C, 40 minutesminutes

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 A passive REM counter designed to counter designed to host at its centre a host at its centre a CR39+BE10 CR39+BE10 detector has been detector has been designed.designed.

The detector has The detector has been calibrated at been calibrated at Politecnico di Politecnico di Milano, with a Pu-Milano, with a Pu-Be source.Be source.

Using an enriched Using an enriched boron converter its boron converter its sensitivity is 7 sensitivity is 7 tracks/cmtracks/cm22Sv.Sv.

The background, The background, due to the due to the background background reduction reduction algorythm used, is algorythm used, is 3 3 ±3 tracks/cm2 .±3 tracks/cm2 .

This implies a LDL This implies a LDL of of 2 2 Sv. Sv.

The detector has been tested The detector has been tested in high energy fields both at in high energy fields both at

GSI Cave A and at CERFGSI Cave A and at CERF At GSI high energy neutrons are At GSI high energy neutrons are

generated by 400 MeV/u C ions generated by 400 MeV/u C ions impinging on a carbon target.impinging on a carbon target.

Measurements have been made Measurements have been made inside the ernty maze and out of the inside the ernty maze and out of the cave shielding, with dose rates cave shielding, with dose rates ranging between 2-40 ranging between 2-40 Sv/h.Sv/h.

The measurements have been carried out in the The measurements have been carried out in the frame of the CONRAD project, involving several frame of the CONRAD project, involving several european laboratories. 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 CERFMeasurements at CERFCERN-EU reference fieldCERN-EU reference field

The high energy neutrons are obtained by 150 Gev protons impinging on metal targets (Cu-Al).

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 The detector has a high uncertainty (if compared to active REM counters), because ofcompared to active REM counters), because of

Poisson Poisson uncertaintyuncertainty: 7 tracks/cm: 7 tracks/cm22μμSv mean Sv mean that a 10 that a 10 μμSv measurement is made with Sv measurement is made with 7070±8±8 tracks, that means a 12% uncertainty. tracks, that means a 12% uncertainty.This is unavoidableThis is unavoidable

Lot uncertainty: the tracks are formed at the Lot uncertainty: the tracks are formed at the very surface of the detector. Different lot of very surface of the detector. Different lot of detectors may have slightly different detectors may have slightly different properties in the first microns from the properties in the first microns from the surface, that can cause a systematic shift of surface, that can cause a systematic shift of the mesurements (up to 20%). This can be the mesurements (up to 20%). This can be avoided calibrating every lot of CR39.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.

The method is very sensitive, and it can provide The method is very sensitive, and it can provide reliable measurements even with integral doses reliable measurements even with integral doses down to 10 down to 10 μμSv.Sv.The LDL is in the order of 2 The LDL is in the order of 2 μμSv.Sv.

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

It is especially suitable for routine area It is especially suitable for routine area environmental monitoring where a large environmental monitoring where a large number of measurements point are needed (es. number of measurements point are needed (es. large plants), or where a large active large plants), or where a large active environmental neutron monitoring system is not environmental neutron monitoring system is not justified (es. conventianal radiotherapy centres)justified (es. conventianal radiotherapy centres)