Dose measurements with CR-39 detectors at the CERF reference facility at CERN

doi:10.1016/j.radmeas.2014.04.010

Radiation Measurements

Volume 71, December 2014, Pages 502-504

https://doi.org/10.1016/j.radmeas.2014.04.010 Get rights and content

Highlights

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Characterization of a personal neutron dosimeter based on CR39 PADC.
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We develop a response function based on the average LET calculation.
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The calibration coefficient is rather constant in a wide energy range.
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We tested the dosimeter at the CERF workplace neutron field.
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The results are very consistent with previous characterizations.

Abstract

A passive neutron dosimeter based on CR-39 track detectors has been successfully tested at the CERF facility at CERN. Former calibration campaigns in monoenergetic and quasi-monoenergetic neutron beams have shown an almost energy independent response function of the detector in the energy range 0.5 MeV–100 MeV. This feature allows to obtain a “rem counter like” personal neutron detector, where a single calibration coefficient is valid for a wide energy range. In order to confirm his feature the detector has been tested in the CERF facility at CERN that generates a workplace neutron field similar to the one encountered at commercial flight altitude. The results show a very good agreement with the reference value permitting to conclude that the detectors have a good reliability also for this kind of radiation field.

Introduction

CR-39 track detectors are widely used for neutron dosimetry in the nuclear industry, in space and around medical and research accelerators. However one important downside of this technique is the energy dependence of the response function. This is caused mainly by the analysis algorithm that correlates the track density produced by the secondary heavy charged particles to the neutron dose, both H_p(10) and H*(10) (Trompier et al., 2013). A novel approach has been recently proposed (Caresana et al., 2013). In this case the detector response is calculated starting from a measurement of the average LET produced by secondary heavy charged particles. The average LET distribution is folded with the ICRP74 (ICRP, 1996) quality factor Q(LET) and the resulting H_Cr is the detector response expressed in mSv. H_Cr underestimates the actual reference dose by a factor about 2, mainly because of the limit angle that forbids the detection of particles impinging with a high dip angle (Caresana et al., 2010). This method has been tested with monoenergetic and quasi-monoenergetic neutron fields and it was demonstrated that the underestimation is fairly constant in the energy range 0.5 MeV to 100 MeV with a dimensionless calibration coefficient C_c = 1.85 ± 0.3 (Caresana et al., 2013).

In order to check if the value of the calibration coefficient is also maintained in workplace fields, it was decided to irradiate sixteen detectors at the CERF facility at CERN (Mitaroff and Silari, 2002) at two distinct positions. The radiation field outside the shielding is similar to the neutron component of the field produced by cosmic rays interacting with the atmosphere at high altitudes, and to the typical workplace neutron fields in accelerator environments, giving indications about the capability of the detector of measuring the neutron dose on board of commercial aircrafts.

Section snippets

Material and methods

A sketch of the CERF facility is shown in Fig. 1a. A mixed protons, pions and kaons primary beam with a momentum of 120 GeV/c impinges on a copper target located below a concrete roof. The neutron spectrum in the sixteen positions on the concrete roof (see Fig. 1b) is characterized in terms of energy distribution (see Fig. 2a) and in terms of neutron dose. The reference dose is measured using an air-filled precision ionisation chamber (PIC) at atmospheric pressure placed in the beam upstream of

Results and discussion

In Fig. 3, an example of average an LET distribution is shown. The x-axis plots the LET_nc where the suffix “nc” stands for “Nuclear track detector CR-39” indicating that it is not the actual LET but an approximation deriving by the measurement procedure that averages the energy deposition on a particle's path in the order of tens of μm.

The signal produced on the track detector is both due to recoil protons and heavy charged particles. Protons are produced by (n,p) reactions in the PMMA due to

Conclusion

The dosimeter, that was calibrated in monoenergetic fields of various energies, at the PTB and Ithemba labs (Caresana et al., 2013), proved effective in measuring the dose in a workplace field having a significant high-energy component. A slight underestimation in one of the two measuring points can be ascribed to the angular dependence of the detector response that still has to be investigated both with measurements and Monte Carlo simulations.

References (6)

M. Caresana
Study of a radiator degrader CR39 based neutron spectrometer
Nucl. Instrum. Meth. Phys. Res.
(2010)
M. Caresana
Determination of LET in PADC detectors through the measurement of track parameters
Nucl. Instrum. Meth. Phys. Res.
(2012)
M. Caresana
Personal and Environmental Dosimetry with a Dosimeter Based on CR-39 SSNTD in Quasi-monoenergetic Neutron Field
(2013)

There are more references available in the full text version of this article.

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The response in terms of personal dose equivalent of a passive fast neutron dosimeter composed of a CR-39 detector coupled with a $1$ -cm-thick PMMA radiator is investigated in monochromatic neutron fields via simulations and experiments. In particular, the simulations were performed through the Monte Carlo FLUKA code (version 2011.2x.5), while the experimental fields were produced at the AMANDE facility (IRSN, France). The dosimeter response is studied with respect to both the energy and the angle of incidence of the radiation field on the device. The simulated response shows a quite stationary trend, that is, minimal variation across a wide energy range, also for different irradiation angles. The agreement between simulated and experimental responses indicates the simple and cheap dosimeter presented as promising for personal dosimetry applications in practical situations.
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CR-39 detectors have been irradiated with both ²⁴¹Am alpha particles and protons in the energy range of 1.0–5.5 MeV and 0.8–2.8 MeV, respectively. The proton irradiation has been achieved through two different techniques: Rutherford Backscattering (RB) and Nuclear Reaction (NR). The detectors were chemically etched in a hot KOH solution for periods ranging from 0.5 h to 3 h for alpha particles and 2.5 to 5.5 h for protons. The track diameter variation as a function of particles energies and etching time has been studied. A new predictive model based on experimental track parameters has been developed for the studied particles and for variable bulk track etching velocities. This predictive model is based on a single parameter $β$ determined by fitting our experimental data.
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