Gunter Pretzsch

Investigation of the radial pore-etching rate in a plastic track detector as a function of the local damage density around the ion path

Abstract An attempt is made to define the sensitivity of plastic track detectors on the basis of a new concept. The main feature of this (micro)concept is the relation between the radial etching rate of a heavy ion track and the local ion damage density by means of a continuous function. The energy dose depending on the radial distance from the ion path is taken as a measure of the local damage density. The corresponding etching rate has been measured by the conductometric method. The sharp rise of the radial etching rate with increasing dose explains the threshold character of the track registration and confirms the applicability of the existing threshold registration criteria for practical purposes. The dependence of the radial etching rate on the radial dose can be described by the multi-target model which explains the preferential etching by the collective activation of a given number of sensitive elements. The lower slope of the corresponding curve for gamma radiation demonstrates the substantial influence of the dose rate on this collective activation process.

Theoretical determination of the neuron detection efficiency of plastic track detectors: I. Theoretical model

Abstract A theoretical model to determine the neutron detection efficiency of organic solid state nuclear track detectors without external radiator is described. The model involves the following calculation steps: production of heavy charged particles within the detector volume, characterization of the charged particles by appropriate physical quantities, application of suitable registration criteria, formation of etch pits. The etch pits formed are described by means of a distribution function which is doubly differential in both diameter and depth of the etch pits. The distribution function serves as the input value for the calculation of the detection efficiency. The detection efficiency is defined as the measured effect per neutron fluence. Hence it depends on the evaluation technique considered. The calculation of the distribution function is carried out for cellulose triacetate. The determination of the concrete detection efficiency using the light microscope and light transmission measurements as the evaluation technique will be described in further publications.

Electret charging utilizing the electret ionization chamber

Abstract Isothermal charge deposition on polymers to form stable electrets by using the two-electret ionization chamber with parallel-plate geometry is reported. One of the electrets exhibits a high starting voltage compared with the second electret to be charged and hence an electric field exists in the chamber volume. Charge carriers produced by irradiation of the sensitive chamber air volume drift in the electric field and cause a charge enhancement of the second electret due to compensation of surface charges of the first electret. This process is finished when both electrets reach the same value and polarity of the surface potential. The method requires no external power supply and allows two electrets of the same surface potential to be produced.

Sensitivity of the electret ionization chamber to fast neutrons

Abstract The electret ionization chamber is an analogue of the common ionization chamber using permanently charged electrodes - electrets The applicability of such electret dosimeters for detecting X and gamma radiation primarily in personnel monitoring has been described by various authors. However, investigations concerning their sensitivity to neutrons are rather rare. In the present paper the sensitivity of a polyethylene-walled electret ionization chamber with parallel plate geometry was determined. Irradiation experiments were made with 3.2 MeV and 14.7 MeV neutrons of normal and parallel incidence. The results are compared with those of a PTFE chamber which exhibits a generally lower sensitivity.

Theoretical determination of the neutron detection efficiency of plastic track detectors: II. Comparison of theoretical and experimental results for light microscope evaluation

Abstract The neutron detection efficiency of organic solid state nuclear track detectors without external radiators was calculated for evaluating the etched detector foils under the light microscope. The calculation procedure is based on a previously determined distribution function of the etch pits, doubly differential in pit diameter and depth and on the description of the limits for track observation according to the resolving power of the optical microscope. The detection efficiency is defined as the observable track density per neutron fluence. Theoretical results are quoted for various microscope magnifications depending on the neutron energy, the angle of incidence and the thickness of the layer removal. The comparison with experimental data shows a good agreement. Results for the mean track diameter and the mean depth depending on the above-mentioned parameters are also given.

Neutron sensitivity of a polyethylene-walled electret ionization chamber

Abstract The electret ionization chamber is an analogue of the common ionization chamber using permanently charged electrodes - electrets. The application of such electret dosimeters for detecting X and gamma radiation primarily in personnel monitoring has been established. In the present paper the neutron sensitivity of a polyethylene-walled electret ionization chamber with parallel plate geometry was determined experimentally and theoretically for various interelectrode spacing distances ranging from 0.2 to 5.0 cm for neutrons of normal incidence up to 15 MeV. The calculation model is based on the ionization yield of recoil protons in the air-filled cavity originating in the chamber wall by elastic neutron scattering. The sensitivity as a function of neutron energy passes into saturation at a given energy which depends upon cavity size. The neutron sensitivity depending on the spacing distance also reaches a nearly constant value contrary to the gamma sensitivity. Comparison of calculated and experimental values shows a good agreement and thus the theoretical model is confirmed.

Detection properties of the electret pulse chamber

Abstract The detection properties of the electret ionization chamber in the pulse mode are described for an air-filled chamber with parallel plate geometry. The chamber consists of an one-sided metallized PTFE electret connected with a charge sensitive preamplifier. Irradiation of the air volume with α-particles as well as of a polyethylene-walled chamber with fast neutrons gives rise to pulse generation. The charge carriers generated in the air volume are attracted by the electric field of the electret and collected on the electret surface lowering its surface potential, initially charged up to 2 kV. The pulse rate decreases nearly proportionally with the electret voltage as a function of increasing irradiation time. Neither gamma nor β-radiation caused pulses but lowered the electret voltage.

Theoretical determination of the neutron detection efficiency of plastic track detectors: III: Detector evaluation using light transmission measurement

Abstract The dependence on the neutron energy, the angle of incidence and the fluence of neutron detection efficiency of organic solid state nuclear track detectors without external radiator for evaluating the etched detector foils by light transmission measurement was calculated. The calculations are based on previously determined distribution function which characterizes the frequency of the etch pits according to their diameters and depths. Furthermore, the evaluation process was described by an appropriate model. The comparison between theoretical values and experimental data, obtained with a specially constructed light transmission measuring device, shows good agreement. From the fluence characteristic, which was described by an analytical function, conditions for the upper and the lower registration limits of the fluence were derived. Moreover, the fluence characteristic allowed the average effective light scattering area of the etch pits to be determined.