W. Enge

Revision of the concept of registration threshold in plastic track detectors

Abstract Response curves ( V  V T / V B versus REL) of various plastic track detectors (CN, CA, PC, PET) were determined in the region of relatively low etching rate ratios V . Comparative investigations made it clear that these detectors are more sensitive than has hitherto been supposed and the registration threshold concept needs revision. It was found that for most of the commercial plastics the V (REL) curves can be well described by the relation V = 1 + α REL β , where the power index is about 3 within a limit ±10% for pure materials. With CN the situation proved to be more complex in the presence of relatively large amounts of camphor in the matrix of the detector. For the interpretation of the observed shape of the V (REL) curves a theoretical model (similar to that which is used to describe survival curves of irradiated biological objects) was proposed. Experiments performed for a better understanding of the nature of the radiation-damage process in plastics yielded a simple relation REL = const·√ D between the restricted energy loss of nuclear particles and the volume dose D deposited by accelerated electron beams which produced equivalent chemical etchability in the irradiated plastics. The theoretical description of radiation-induced effects in biological objects (as proposed by Kellerer and Rossi) results in a relation between the LET value of nuclear particles and the absorbed dose which is similar to our formula. Finally, it is illustrated that the observed response of PC detectors to protons and alpha-particles can be well explained without supposing any kind of registration threshold.

Etching behaviour of the Lexan polycarbonate plastic detector

Abstract The etching mechanism of Lexan Polycarbonate plastic - represented by the bulk etching rate ν b and the track etching ν t - was studied under the variation of the etching parameters like temperature T , etchants, normalities N , and mechanical stirring. The influence of the etchants LiOH, NaOH and KOH at various normalities (1N–13N) and different temperatures (50°–80°C) was analyzed. Using NaOH the bulk etching rate as well as the track etching rate are described by empirical function ν b (N, T) and ν t ( REL , N , T ). The normalized track etching rate ν ( REL ) = ν t ( REL , N , T )/ ν b ( N , T ) depends only on the restricted energy loss REL and not on the temperature or normality. The charge and isotopic resolution do not change with different etching condition of NaOH over a wide range (4N–8N, 50°–80°C). At 70°C the stirring of the etchant has no effect on the track-etching rate, and inside of an etch cone the etchant is saturated with etch products.

Etching behaviour of a cellulose nitrate plastic detector under various etching conditions

Abstract To study the etching mechanism of cellulose nitrate (Daicel, Japan) the etching parameters have been varied. Different etchants (LiOH, NaOH, KOH) at various concentrations between 1 and 15 normality have been used at different temperatures between 15°C and 50°C. The effects of stirring the etchants and interrupting the etching process have been studied. The bulk etching behaviour of cellulose nitrate depends on the normality, the temperature and the stirring of the etchant. The track etching rate of cellulose nitrate is independent of mechanical stirring. The threshold of the detector and the charge and isotopic resolution were determined for the different track etching rates. A higher resolution is accompanied by a higher threshold.

Effect of gamma irradiation on bulk etch rate of CR-39

Abstract The molarity and temperature dependences of the bulk etch rate of CR-39 have been restudied with NaOH etchant over the molarity range 0–10 M and temperature range 40–75°C. The exponential law fits the molarity dependence better than the power law. The linear relationship between the bulk etch rate of CR-39 and the gamma dose over the dose range 0–10 5 Gy obtained from a 60 Co gamma-ray source suggests the use of CR-39 as a simple gamma dosimeter.

Oxygen effect on the etch rate in CR-39 plastic detector

Abstract CR-39 plastic detector sheets ( about 250 μm thick, produced by Pershore Mouldings, England, having a 32 h curing cycle and no DOP ) were irradiated with fission fragments of a 252 Cf-source and 16.3 MeV/nuc Fe-ions at the GSI, Darmstadt, W.-Germany. Afterwards they were stored for about 100 h in an Oxygen atmosphere at different pressures ( 1 and 30 at ) and temperatures ( 20°C and 70°C ). Both, the bulk v b and the track etch rate v t (6n NaOH, 70°C ) increased by a factor of about 3.5, when stored at 30 at and 70°C for 100 h. Thus the normalized track etch rate v t /v b remains about constant. No improvement of the charge resolution in particle identification can be obtained. At 20°C room temperature and 30 at pressure no increase was observed.

A SPATIAL TRACK FORMATION MODEL AND ITS USE FOR CALCULATING ETCH-PIT PARAMETERS OF LIGHT NUCLEI

Abstract A generalized geometrical model of etch-pit formation in three dimensions is presented for nuclear particles entering isotropic solids at arbitrary angles of incidence. With this model one can calculate the relations between any particle parameter (Z=charge, M=mass, R=range, θ=angle of incidence) and etching or track parameter (h=removed detector layer, L=track length, d=track diameter, etch-pit profile and contour) for track etching rates varying monotonically along the trajectory of particles. Using a computer algorithm, calculations have been performed to study identification problems of nuclei of Z=1–8 registered in a stack of polycarbonate sheets. For these calculations the etching rate ratio vs residual range curves were parametrized with a form of V −1 (R) = 1 − Ωa i exp (−b i R) which does not involve the existence of a threshold for track registration. Particular attention was paid to the study of the evolution of etch-pit sizes for relatively high values of h. For this case, data are presented for the charge and isotope resolving power of the identification methods based on the relations L(R) or d(R). Calculations were also made to show the effect of the relative (parallel and opposite) orientations between the directions of track etching and particle speed on etch-pit evolution. These studies offered new identification methods based on the determination of the curves L(parallel) vs L(opposite) and d(parallel) vs d(opposite), respectively.

Some new aspects on etching of CR-39 plastic detector

Abstract Pershore Mouldings CR-39 plastic sheets (∼250μm, without DOP, 32 h curing cycle) were irradiated with 12 C-ions of 26 MeV/nuc at the Hahn-Meitner Institute in Berlin in order to investigate the etching behaviour. The influence of NaOH etchant at various normalities (4n – 8n) and different temperatures (30°C – 80°C) was analyzed. The bulk etch rate as well as the track etch rate can be fitted by analytical functions v b (n,T) and v t (REL,n,T). The normalized track etch rate v(REL) = v t (REL,n,T)/v b (n,T) in this region only depends on the restricted energy loss REL(200eV). Thus the charge resolution does not change with different etching conditions of NaOH in the range of our measurements (4n – 8n,60°C – 80°C). At 70°C the stirring of the etchant and the concentration of its Benax admixture have no effect on the bulk etch rate.

ELECTROLYTICAL STUDIES OF SUBMICROSCOPIC NUCLEAR TRACKS IN PLASTIC DETECTORS

ABSTRACT The track etching properties are studied in the submicroscopic diameter range of 30–600A in 10μm Makrofol KG foils irradiated with252Cf fission fragments. During the etching process in 6n-NaOH the resistance of the foil is monitored starting from 1013ω down to about 108ω after the break-through of an individual track. Curves of the time dependent conductance of a single hole and the transversal track etching rate are presented. From these measurements the latent track diameter can be estimated to 100-A.

Effects on ontogenesis of Carausius morosus hit by cosmic heavy ions

Among the biological problems that arise in long duration spaceflights, the effects of weightlessness and ionizing radiation appear to be the two main risk factors. Eggs of the stick insect Carausius morosus were exposed to spaceflight conditions during the 12.56 day Biosatellite mission Cosmos 1887. Five different ages were used, representing different sensitivities to radiation and different capacities for regeneration. During spaceflight the eggs continued their development. Already, in the Spacelab D1 mission in 1985, it has been shown that microgravity leads to a reduced hatching rate of eggs exposed during the early steps of development. When the eggs were hit by a heavy ion, a further but not significant reduction of the hatching rate was observed. Hatching was normal for eggs which were exposed on a 1 g reference centrifuge in space. Heavy ion hits caused body anomalies. The combined action of heavy ions and microgravity resulted in an unexpectedly high rate of anomalies. In the experiment on Cosmos 1887 these results were confirmed. Studies on the embryonic development before hatching showed no major difference between flight and ground control specimen, neither in speed of development nor in morphological anomalies. Hatching therefore seems to be the critical point in insect ontogenesis.

The diffusion-etch-model Part I : Proposal of a new two phase track developing model

ABSTRACT In our track developing model two processes are regarded to create the latent track: first the well-known process of the energy deposit along the path of the ion and second the newly proposed process of additional track formation in the region of the already existent latent track through NaOH which has diffused into the detector material before the real etching process had started. A first order mathematical description of the model and a discussion of the experimental implications will be given.