The calibration of the CR39 nuclear track detector

The calibration aims to establish the dependence of the response of the CR39 nuclear track detector on the energy losses of charged particles with different velocities and different charges and at determining the contribution to the response of the nuclear energy loss [1][2].
Both types of the Intercast CR39, EN3 and L6, have been tested and calibrated with relativistic heavy ions:

1. oxygen nuclei of 16 GeV/nucleon at Brookhaven (1988),
2. neon nuclei of 585 MeV/nucleon at Berkeley in 1990,
3. silicon nuclei of 14.5 GeV/nucleon at Brookhaven (1990),
4. sulphur nuclei of 200 GeV/nucleon at CERN (1990)
5. gold nuclei of 11.3 GeV/nucleon at Brookhaven (1992),
6. lead nuclei of 153.8 GeV/nucleon at CERN (1994).

After exposure the samples were etched in a 6.0 N NaOH water solution at 70 ° C for 45 h. One sample exposed to gold nuclei was etched at 40 ° C for 265 h. The etched tracks were measured automatically with the Elbek [3] image analyser system.

Samples of CR39 were also exposed to low velocity ions: to protons and deuterons of 50, 100 and 200 keV and to beryllium and silicon ions with kinetic energies ranging from 100 to 400 keV. The exposures were done by the IICO Co. (Santa Clara, California).
Other samples were irradiated at the Van de Graaff accelerator in Laboratori Nazionali di Legnaro, Italy, with helium ions of 6 MeV and 10 MeV at normal incidence. After exposure the samples were etched in a 6.0 N NaOH water solution at 40 ° C for different etching times, from 1 to 14 hours.

The response of the CR39 nuclear track detector has been measured by the so-called reduced etch ratio p of the track etching rate, vT, to the bulk etching rate, vB, p=vT/vB. The measured p-values are shown in the Calibration Curve figure. The p-values for low velocity ions and follow the relativistic calibration data, thus indicating that, within errors, the dependence on REL of the CR39 response is the same at all velocities.

The calibration with lead ions is in progress. Preliminary results indicate no relevant ageing effect for the CR39 made more than 5 years ago.

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REFERENCES
[1] S. Ahlen et al., (MACRO Collaboration), Improvements of the CR39 polymer for the MACRO experiment at the Gran Sasso Laboratory, Nucl.Tracks Radiat.Meas. 19 (1991) 641.
[2] P. Baiocchi et al., Calibration with relativistic and low velocity ions of a CR39 nuclear track detector, Radiation Measurements 25 (1995) 145.
[3] A. Noll et al., Nucle. Tracks Radiat. Meas. 15 (1988) 265.

For information : intercast@relay1.it.net

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