US3709692A

US3709692A - Silver chloride monocrystal doped with cadmium and low concentration of lead

Info

Publication number: US3709692A
Application number: US00085285A
Authority: US
Inventors: G Haase; E Schopper
Original assignee: Agfa Gevaert AG
Current assignee: Agfa Gevaert AG
Priority date: 1969-11-14
Filing date: 1970-10-29
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09
Also published as: FR2067309B1; FR2067309A1; BE758734A; GB1302785A; DE1957313A1

Abstract

The photographic properties of silver halide monocrystal particle-track detectors which contain cadmium are highly improved by an additional content of lead-II ions.

Description

United States Patent 1191 1111 3,799,692 Haase et al. 1 1 Jan. 9, W73

[54] SILVER CHLORIDE MONOCRYSTAL [56] References Cited 3,047,392 7/1962 Scott ..96/94 BF [75] Inventors grx figssz} ggfi zg 3,219,449 11/1965 Saxe et al ..96/94 BF nus both of y 3,362,797 l/l968 Shaskolskaja ..96/94 BF [73] Assignee: Agia-Gevaert Aktiengesellschaft, primary E i j Travis Brown Leverkusen, Germany Assistant ExaminerW0n H. Louie, Jr. 221 Filed: 06. 29, 1970 A1wmey-C9nn9lly & Hutz [21] Appl. No.: 85,285

[57] ABSTRACT [30] Foreign Application Priority Data The photographic properties of silver halide Nov 14 1969 German P 19 57 313 8 monocrystal particle-track detectors which contain y cadmium are highly improved by an additional con- 52 us. c1. ..96/l08, 96/27 E, 96/94 BF, tem of 252/408, 96/452 [51] Int. Cl. ..G03c 1/28 3 Claims, No Drawings [58] Field of Search ..96/94 BF, 108; 23/85, 87; 252/408 SILVER CHLORIDE MONOCRYSTAL DOPED WITH CADMIUM AND LOW CONCENTRATION OF LEAD The present invention relates to particle track detectors consisting of silver halide monocrystals which are improved in their sensitivity and have their fogging reduced by the addition of certain doping agents.

In the investigation of atomic particles, especially in modern heavy ion physics, solid state particle track detectors have achieved great importance for the detection of tracks of ionizing particles. Such particle track detectors must meet certain requirements, especially if they are to be used for quantitative measurements, and, in particular, the interaction of the particle under investigation with the solid must develop in a characteristic and accurately reproducible manner.

The particle track must provide as much information as possible about the particle. It must be able to be rapidly and easily interpreted.

Since the defects which an ionizing particle produces in the solid are submicroscopic in size, mechanism for amplifying the track must be available for purposes of photo-optical interpretation, e.g. for rendering the track visible. The defects produced in the solid by the ionizing particles represent the latent image of the particle track, which is developed by the amplification mechanism. The more the track discloses details characteristic of the particles, the better is the detector.

Two amplification mechanisms have attained practical importance:

1. Selective etching of the solid along the particle track.

2. Deposition of a new phase along the track.

Etching has become important inter alia in the case of mica and some inorganic glasses and, in particular, organic high polymers. The selective etching process along the particle track is mainly based on the fact that bonds dissolved along the track considerably facilitate the etching process. Numerous difficulties, however, arise in the etching process, which considerably restrict its utility.

The most serious disadvantage of the etching process is that valuable details are often lost, especially in the case of long particle tracks, because the etching medium must travel from the outside through the solid along the track, and the etching channels available for this are very narrow, with the result that the etching medium is often not able to penetrate sufficiently deeply into the solid state detector. It is, therefore, in most cases not possible to amplify discontinuous particle tracks by this method.

It is also known to detect tracks of ionizing particles in silver chloride monocrystals. In this type of detector, a new phase is preferably deposited along the particle track. In the case of silver halide mono-crystals, this new phase consists substantially of silver.

The silver chloride monocrystals are superior to the above-mentioned solid state particle track detectors, in which the particle tracks must be amplified by an etching process, especially in that in silver chloride monocrystals the amplifying and development process can be carried out very simply and rapidly. The amplification process consists in a uniform exposure of the crystal platelets, in which the particle track was recorded, to high energy light, preferably UV light.

The development process can be explained as follows:

The exposure to light causes electron-defect electron pairs to be produced in the crystals. The electrons are trapped along the particle track in interchange with silver ions from the disturbed regions. The track is in this way stabilised and then amplified. This process is, in principle, comparable to the elementary photographic process. The original track is the latent image of the track and the amplification then corresponds to the photographic development.

The disadvantage of these silver chloride monocrystal detectors was, in the first instance, their inadequate reproducibility. This disadvantage was overcome by using highly purified silver chloride for the production of the detectors. It is known that such silver chloride is in itself insensitive and useless for the production of detectors, but by the addition of certain substances, these silver chloride crystals can be rendered sensitive to ionizing particles. This was achieved, e.g. by the addition of cadmium or lead. In this connection the article by K. Breuer, G. Haase and E. Schopper in British Journal of Applied Physics, 18 (1967) page 1824 et seq. and the publication by K. Breuer, E. Schopper, G. Haase and F. Zorgiebel in Phot. Korrespondenz 104 (1968) page 76 et seq.are to be noted. The silver chloride crystals doped as described above are sufficiently sensitive for many purposes. They are advantageous also in that they do not register 'y-rays, X-rays and electrons, so that these rays do not produce an interfering background.

For more accurate quantitative measurements on tracks of ionizing particles, the silver chloride monocrystals doped with cadmium alone, which were, as such, very promising, were still in need of improvement both with regard to their sensitivity and especially with regard to the background, i.e. the signal-to-noiseratio. This background, which interferes: with the interpretation of the particle tracks, is due mainly to:

I. lattice defects which are already present in the crystals from the start, i.e. even before the irradiation of the particles, and which can never be completely avoided, especially displacements or small angle gain boundaries of general substructures which, rather like the lattice defects produced by irradiation of the particles, are decorated with silver along the particle tracks in the amplification process;

2. silver particles which are statistically distributed in the crystal and which have been formed by photolysis during the amplification process (print-out);

3. deposits which are formed in the course of production of the crystals and which, owing to the limited solubility of cadmium in silver halide, occur especially in silver halide monocrystals doped with high concentrations of cadmium, and which cause optical clouding which can constitute a serious interference, especially in fairly thick crystals.

It is among the objects of the present invention to provide improved silver halide particle track detectors which have higher sensitivity to light and a reduced background.

We now have found silver halide monocrystal detectors doped with cadmium for recording tracks of ionizing particles, which detectors in addition contain lead ions in quantities of up to ppm as a second doping agent.

This small addition of lead ions in general lead-II ions substantially suppresses the interfering background while the sensitivity to light remains the same, so that substantially better particle track images which can be more reliably interpreted are obtained. It is to be assumed that the small additional doping with lead causes the above-mentioned developable defects which are already present in the monocrystal, such as crystal shift, etc., to become insensitive to such an extent that they no longer form a background during the amplification process by exposure to UV light.

The effect of the lead addition according to the invention is particularly unexpected in view of the fact that it was known that the sensitivity of silver halide crystals towardsionizing particles could beincreased by the addition of lead and that a relatively powerful background which prevents quantitative measurements occurs also in those detectors which are doped with lead alone. It could, therefore, not be predicted that suppression of the background could be achieved by the addition of small amounts of lead to cadmiumdoped silver chloride monocrystals.

By means of the particle track detectors which are doped in accordance with the present invention, ion tracks can be recorded without an interfering background. The concentration of cadmium may vary within wide limits. It depends primarily on the nature of the ionizing particles which are required to be detected with any given detector. Concentrations of about 50 ppm (parts per million) up to about 1 percent by weight of cadmium, based on the weight of the silver halide, preferably silver chloride, have generally been found to be sufficient. If the amount of cadmium added is small, only decay products and heavy ions can be detected. In this way, an interfering background due to the effect of light particles can be largely avoided. At cadmium concentrations of over 0.1 percent by weight, practically all the ionizing particles, even the lighter ones, are recorded.

The required quantity of lead ions is 5 to 100 ppm, preferably 5 to ppm.

The particle track detectors according to the present invention may be used for determining particle data, for the investigation of particle reactions and nuclear fissions, the investigation of decay mechanisms including those of superheavy nuclei, the identification of isotopes of high energy ions and the investigation of isotope compositions of solar radiation or of cosmic radiation to determine the sources of this radiation. These detectors are especially suitable for recording tracks of heavy ions.

The track of ionizing particles can be amplified in the detectors according to the present invention in the usual manner by uniform exposure to shortwave light, especially UV light. Extremely sharply defined tracks can thus be obtained on a clear background.

In this respect, the detectors are superior to conventional photographic emulsions for recording nuclear tracks (nuclear track emulsions). These emulsion materials consist of a silver halide gelatine emulsion layer which has a high power of resolution, on a layer support. With these photographic emulsions, it is generally not possible to obtain such sharp particle tracks as in the detectors according to the present invention. The photographic emulsions, moreover, generally have a more strongly interfering background since they are also sensitive to 'y-rays, X-rays and electrons.

In the detectors according to the present invention, tracks of ionizing particles can be amplified along their whole length for practically any given length, even if the tracks are discontinuous, i.e. if between the crystal regions of high interference, which are produced by the ionizing particles running through them, there are crystal regions which are undisturbed or relatively little disturbed, in which the track is interrupted. This follows inevitably from the nature of the amplification process since in the amplification process which occurs inside the volume, electrons and silver ions are exchangeably deposited wherever lattice defects have been produced by the ionizing particles running through the volume. In this respect, the detectors according to the present invention are generally superior to detectors in which the track amplification is produced by an etching process. The etching process starts at the surface of the detector, where the ionizing particle has entered the crystal, and continues along the track of the particle into the interior of the crystal, and fresh etching solution must be supplied along the channel already formed by the etching process. In the case of discontinuous particle tracks, the etching process is liable to stop at the end of a track section because the etching solution then cannot penetrate sufficiently rapidly-the adjacent undisturbed region of the crystal, so that the following sections of the track which are not continuous with the previous track can no longer be amplified. In the case of discontinuous particle tracks, a less disturbed or even undisturbed crystal region between two track sections may occasionally be penetrated by the etching solution if the time allowed for the etching solution to act is considerably increased. In that case, however, the etching solution also continues to act during this period in that portion of the track which was etched first and which has therefore been amplified, with the result that this first section of track becomes greatly increased in width and may acquire a pronounced cone shape. This, however, seriously impairs the reproducibility of the track and the accuracy of the interpretation. The detectors according to the present invention are completely free from such disadvantages.

The possibility provided by a silver halide monocrystal detectors according to the present invention of producing an amplification of uniform sharpness and high reproducibility even of those particle tracks which start at some depth within the detector opens up fields of application for these detectors in which other solid state particle-track detectors hitherto known could not be used with the same assurance and accuracy. An example of this is the study of the development of the decay process with time. If amplification of the particle tracks is carried out first at a point in time t, and then at a point in time t,, it is possible to determine which tracks have been added during the time interval t, 1,, i.e. which new decay processes have taken place inside the detector during the time interval t, t

The amplification process by uniform exposure of the detectors to shortwave light is characterized by its simplicity and freedom from interference. After the recording of the particle track, the detectors are not exposed to any liquids, so that any disturbances which might be caused by liquids are avoided. It is worth mentioning here by way of comparison, the sensitivity of the etching methods in this respect and the swelling and distortion phenomena which occur in the conventional photographic processing of nuclear track emulsions.

EXAMPLE An aqueous solution of cadmium chloride (CdCl '2.5 H O p.a.) is added in a pipette to pulverulent silver chloride which has a degree of purity of 99.999 percent, resulting in a silver chloride which has a cadmium content of about 600 ppm. The mixture is dried in the pipette in a drying cupboard.

An aqueous solution of lead chloride (PbCl p.a.) is then added to this mixture in the same manner, resulting in a silver chloride which has the above cadmium content and in addition a lead content of about ppm. The mixture is again dried in the pipette in a drying cupboard.

The silver chloride doped with cadmium and lead is melted in the pipette and the melt is placed between two quartz glass platelets which are heated to about 550C and the distance between which is fixed at about 200 p. by rods of quartz glass. On cooling, a polycrystalline silver chloride platelet doped with cadmium and lead is obtained.

The sandwich, consisting of the two quartz glass platelets with the silver chloride platelet between them is placed in a horizontal quartz glass tube which, after being evacuated, is filled with nitrogen to a pressure of 400 mm Hg. A tube furnace is then passed over the quartz glass tube at such a temperature and at such a rate that the polycrystalline silver chloride platelet doped with cadmium and lead is converted in a known manner, by a melting process, into a monocrystal which can be dissolved from the quartz platelets by dipping the sandwich into water.

After irradiation of the ionizing particles which are to be investigated and whose tracks are to be recorded, the silver chloride monocrystal doped with cadmium and lead is uniformly exposed to a. Xenon high pressure lamp, a filter being interposed between the source of light and the detector so that only a narrow range of wavelengths in the region of 417 millimicrons comes into play. The intensity of the shortwave light irradiating the specimen is about 10 quanta per cm per second. The exposure time is about 20 to 30 minutes.

We claim:

1. In a silver chloride monocrystal for detecting the track of ionizing particles and containing 20ppm to 1% by weight of cadmium as a doping agent that increases its tracking sensitivity, the improvement according to which the crystal also contains, as a second doping agent that reduces background interference with the track detection, lead-(II) ions in a concentration of 5 ppm to ppm.

2. The combination of claim 1 in which the cadmium doping is in the form of cadmium-(II) ions present in a concentration of 100 ppm to 5000 ppm.

3. The combination of claim 1 in which the concentration of lead ions is 5 to 20 ppm.

Claims

2. The combination of claim 1 in which the cadmium doping is in the form of cadmium-(II) ions present in a concentration of 100 ppm to 5000 ppm.
3. The combination of claim 1 in which the concentration of lead ions is 5 to 20 ppm.