Nuclear Track Detection最新文献

Induced-fission track densities have been compared on mica external track detectors (2π geometry) and internal mineral surfaces (4π geometry) for the minerals apatite, zircon and sphene. The track-density ratios indicate that the geometry factor relating these two track-registration geometries is indistinguishable from the ideal value of 0·5 (i.e. 2π/4π) for these minerals. External track detectors are not geometrically equivalent to external etching surfaces of a uranium-bearing material. Values significantly higher than 0·5 are caused by a low etching efficiency on the internal mineral surface. The etching efficiency varies with crystallographic orientation in zircon, and may be lowered after thermal annealing in sphene. Etching a mineral surface for spontaneous tracks has no observable effect on the uranium concentration of that surface. These observations have important implications for the use of the external-detector method in fission-track dating.

Cosmic-ray track studies have been carried out in more than 250 fragments of Dhajala meteorite comprising greater than 70% of the recovered mass. In the case of larger fragments (namely, those with mass exceeding 250g) several faces of each fragment have been analysed for track densities. Track densities are low, and fall generally in the range (10³-10⁵)cm^-2, indicating appreciable ablation losses since the cosmic ray exposure age of Dhajala is about 7 m.y. (track measurements were confined to large olivine grains to minimize contributions to tracks due to the fission of uranium and extinct radionuclides). Attempts have been made to deduce information about fragmentation dynamics and the preatmospheric mass/radius of Dhajala, based on the present comprehensive study of track densities in the fragments. Correlations between the position of a fragment in the strewnfield and its track density have provided us with an approximate scenario for the fragmentation/ablation of the meteorite during its atmospheric flight. Observation of minimum track density in the fragments lead to a value of (38±2) cm for the preatmospheric radius of the meteorite. It is estimated from these data that the collection of fragments was made with an overall efficiency of ≈60% and that the ablation amounts to (86.7±2.1)%. Estimated amounts of ablation for shells of different radii are also presented.

In the present study a new and relatively simple method has been developed to measure the ratio of thermally reduced track length to unaffected track length l_s/l_i. The method consists of counting track densities on different parallel planes cut from a single apatite sample, instead of measuring track lengths of a single surface plane; thus the desired ratio of lengths can be derived from ratios of track densities only. This method has been applied to samples of Durango apatite, yielding an averaged ratio of $\text{l}{}_{\mathrm{i}}\text{/}\text{l}{}_{\mathrm{s}}$=1.14±0.06. The result is in good agreement with values we obtained by other independent methods of 1.12±0.05 and 1.13 ±0.05, respectively.

The present method, in addition, can also be used to determine extremely high fission track densities which otherwise would be too high for optical counting.

The possibility of α-track counting by jumping-spark counter in cellulose acetate and polycarbonate nuclear track detectors was studied. A theoretical treatment is presented which predicts the optimum residual thickness of the etched foils in which completely through-etched tracks (i.e., holes) can be obtained for α-particles of various energies and angles of incidence. In agreement with the theoretical prediction, it is shown that a successful spark counting of α-tracks can be performed even in polycarbonate foils. Some counting characteristics, such as counting efficiency vs particle energy at various etched-foil thicknesses, surface spark density produced by electric breakdown in unexposed foils thickness, etc., have been determined. Special attention is given to the spark counting of α-tracks entering thin detectors at right angle. The applicability of the spark-counting technique is demonstrated in angular-distribution measurements of the ²⁷A1 (p,α₀)²⁴Mg nuclear reaction at E_p=1899 keV resonance energy. For this study 15 μm thick Makrofol-G foils and a jumping-spark counter of improved construction have been used.

With the measured etch-rate calibration parameters, the isotope resolution properties of various plastic detectors can be assessed. It is found that, in general, plastics can resolve the isotopes of high-Z particles more readily than those of low-Z particles. For lower-Z particles, a more sensitive detector is required to achieve the same resolution. With the most sensitive detector available, namely cellulose nitrate, the etch-rate calibrations show that the mass number of individual particles with Z as low as 10 can be measured. The properties of different types of plastics and processing procedures, and their suitability for high-precision Z (and A) measurements, are reviewed. Emphasis is placed on using the proper data-reduction techniques and on having an accurate representation of the etch rate as a function of the several parameters involved.

As the etching of a charged-particle track proceeds in a detector, three successive phases can be recognized: the “cone phase”, the “transirion phase”, and the “sphere phase”. Equations are derived to give the values of various useful etch-pit parameters, in particular those connected with the surface openings in the three phases. These values are of use in reconstructing the kinematics of the ionizing particle, such as its charge, mass, energy, and range.

From measurements of the total recordable lengths of fossil tracks and freshly-produced Fe ion tracks in meteoritic hypersthene, the abundance of Fe relative to all VH ions (20 ≦ Z ≦ 28) in the time-averaged cosmic-ray flux is found to be ≈ 0.35. After allowance is made for the effects of fragmentation of nuclei at a depth of ≈ 11 cm within the meteorite, this value is found to be in broad agreement with presentday measurements.

These crystals have been irradiated with a number of heavy ions with a view to calibrating them for the identification of VVH cosmic rays by measurement of track-etch velocity, V_T, as a function of residual range. A good fit to the calibration data is obtained by using a value for the constant K in the primary-ionization equation of 10.5 (±0.5). The relationship between V_T and primary ionization J appears to be approximately linear.