WO2018139954A1 - Sealed isotopic source of fission fragments based on californium-252 and a method of producing same - Google Patents

Abstract

The inventions relate to the field of surface science and may be used in the production of sources of fission fragments with a large area that can be used in applied nuclear physics, e.g. to calibrate and determine the effectiveness of threshold detectors of fission fragments when studying the induced fission of nuclei by neutrons, protons, gamma quanta, etc. in a broad range of energies. A device and a method of producing a sealed isotopic source of fission fragments based on californium-252 consists in depositing californium-252 on an aluminium substrate by desorbing the substance by exposure to its own fission fragments (self-sputtering), and then forming a sealing layer of aluminium oxide (Al₂O₃). The proposed inventions solve the problem of producing a sealed and stable fissile-fragment source that has an energy spectrum of fragments that are separated into heavy and light groups, with minimal consumption of the fissile substance.

Description

 SEALED ISOTOPIC SOURCE OF FISSION SHARES BASED ON CALIFORNIA-252 AND METHOD FOR ITS MANUFACTURE

The invention relates to the field of physical chemistry of the surface and can be used in the manufacture of sources of fission fragments of a large area used in applied nuclear physics. For example, in order to calibrate and determine the effectiveness of threshold fission fragment detectors in the study of stimulated fission of nuclei by neutrons, protons, gamma rays, etc. in a wide energy range, it is necessary to use spontaneous fission fragment sources with a uniform distribution of the active layer over an area of several cm ² and an energy spectrum, with the separation of fragments into heavy and light groups.

 To prevent leakage of the active substance and environmental pollution, sources of radioactive radiation must be sealed and ensure their mechanical strength. It should be noted that fission fragments during braking in a substance lose a unit of length more energy than other types of radioactive radiation (alpha, beta, gamma) and, accordingly, more intensively destroy protective coatings. With a relatively small surface activity of the source (~ 100 divisions per cm), a thin film of alumina (100 μg / cm thick) can serve as a protective coating, which does not pass heavy recoil nuclei and at the same time allows the spectrometric properties of the source to be preserved.

 In the sources of fission fragments under consideration, the spontaneously fissile isotope Cf-252, hereinafter referred to as "California", is used.

A known method of manufacturing sources of fission fragments, in which the deposition of California is carried out by self-spraying the mother layer on a metal substrate (R. C. Gatti, L. Phillips, IIR. Bowman and SG Thompson. "Self-transfer of Cf-252." UCRL-9093 (1960); S. Pauker and NH Steiger-Shafrir. Transfer properties of ²⁵² Cf and their use for source preparation. Nucl. Instr. & Meth. 91 (1971) 557-563).

The disadvantage of this method is that in the absence of sealing, the resulting active layer has weak adhesion to the surface of the substrate, can be easily mechanically removed from the substrate, and can also fade with time under the influence of its own self-dispersion. Closest to the proposed invention are a device and a method of manufacturing a sealed source of fission fragments, in which a layer of gold ~ 1 μg / cm thick with a diameter equal to the diameter of the active spot ~ 3 mm is deposited on the substrate, onto which radiochemically pure California is deposited as a drop and after after drying the droplet, a sealing layer of nickel 1 mg / cm ² thick is applied (. Patent Ru 2207639 (06/27/2003). Bull. 8).

 The disadvantages of this device and method are the limited geometric size of the active spot of California-252, mechanically weak coating with a nickel film, and the relatively low quality of the spectrum of fission fragments, the energy loss of which is about 30 MeV.

 The proposed inventions solve the problem of manufacturing a sealed source of fission fragments having a large active layer area and an energy spectrum of fragments with separation into heavy and light groups, with a minimum consumption of fissile material.

 The proposed device includes an aluminum substrate, a layer of California-252 deposited on the entire surface of the substrate and a sealing film of aluminum oxide formed on top of it.

A method of manufacturing such a source is that a California-252 layer is deposited by desorption of a substance under the action of its own fission fragments (self-spraying) in vacuum from the mother source by scanning on the entire surface of the aluminum substrate, after which a sealing layer of aluminum oxide is formed (A1 ₂ 0 ₃ ) by applying an aluminum layer over a layer of California, calcining the substrate with the deposited layers at a temperature close to the melting temperature of aluminum and subsequent anodic oxidation.

Figure 1 of the drawing shows a diagram of the proposed source device. The source includes an aluminum substrate 1, an active layer of California, occupying almost the entire area of the substrate 2 and a sealing layer of aluminum oxide 3.

 The proposed method of manufacturing a source of fission fragments is carried out in the following sequence.

First, an aluminum foil substrate is prepared, the surface of which is passivated by boiling in concentrated nitric acid. Then, California-252 is deposited onto the surface of the substrate by self-spraying with maternal source in a vacuum. The deposition is carried out within the required area of the active spot by gradually moving the substrate (scan) in order to obtain a uniformly distributed layer and for a time determined by the deposition rate and the required value of the obtained activity. After deposition of the California-252 layer, its total activity, the spectrum of fragments, and the uniformity of the activity distribution are measured using a semiconductor spectrometer.

 Next, the substrate with a layer of California-252 deposited on it is coated with a ~ 100 μg / cm thick aluminum layer by vacuum deposition. After that, the entire composition — the substrate with the deposited layers of California-252 and aluminum — is heated in vacuum to a temperature close to the melting temperature of aluminum (~ 650 ° C) in order to obtain good adhesion of the said layers to the substrate.

 The formation of an oxide film is carried out by the method of anodic oxidation. For this, the substrate is immersed in an electrolytic cell with an ammonium solution, to the electrodes of which a voltage of about 200 volts is applied. At the end of electrolysis, the substrate is washed in distilled water and dried. After the formation of the oxide film, the total activity, the spectrum of alpha particles and fission fragments, and the uniformity of the activity distribution are measured using a semiconductor spectrometer.

 Check the manufactured source for leaks as follows. At a distance of 0.2-0.3 mm from the surface of the source, a clean aluminum foil is placed and the source-foil assembly is placed in a vacuum. After a certain exposure, alpha activity is measured on the surface of the foil in the alpha peak region of California.

 The manufactured source is checked for the mechanical strength of the surface by “swabs” with cotton swabs and subsequent measurements of the alpha activity of the swab.

 During operation of the source, the verification is periodically repeated.

Example.

A 10 ^{6 6} Torr self-spraying method in a 10 ^{6 6} Torr vacuum foil from a parent source weighing ~ 0.5 μg was applied to a substrate of aluminum foil 0.1 mm thick, treated in boiling nitric acid, by applying a 15x15 mm California layer by moving the substrate relative to the parent source.

s A ~ 100 μg / cm thick aluminum layer was deposited by vacuum thermal evaporation on top of a California layer, and then, without violating the vacuum, the substrate with layers of California and deposited aluminum was heated to a temperature close to the melting temperature of aluminum (~ 650 ° C).

 After that, the substrate was placed in an electrolytic cell, the voltage of -200 V was applied to the electrodes of the cell. The electrolysis process was controlled by measuring the current. At the end of electrolysis, the substrate was washed in distilled water and dried.

 In FIG. Figure 2 shows the energy spectra of fission fragments emitted from the active layer deposited in the process of self-atomization of the mother source, before (curve 1) and after the formation of a sealing film of aluminum oxide and drying of the substrate (curve 2).

 The energy spectrum of fission fragments emitted from an active spot without a coating is practically the same as the reference one (P. Wolilfarth et al. Pulse height defect of fission fragment products in a silicon surface barrier detector. Nucl. Instr. & Meth. 140 (1977) 189 -191).

 The shift of the spectrum after the formation of the oxide film relative to the reference one is on average about 7 MeV, which corresponds to the inhibitory energy loss of fragments in the alumina layer with a thickness of about 0.4 μm.

 In FIG. Figure 3 shows source samples made by the above method.

The source leak test was carried out once every 3 months for one year. In each cycle, a clean aluminum collector was placed above the source through a 0.5 mm thick spacer ring. The assembly was heated under vacuum of 10 ^"6 torr for 2 days, then collection was measured on the alpha-spectrometer. Pulses corresponding alpha activity on the surface of collections in the alpha-peak californium-252, at this stage test were observed, which testified to the tightness of the source.

 Periodic measurements of the activity of the source over one year showed a decrease in activity in exact accordance with the half-life of California-252 (2.64 years).

Claims

Claim  1. A sealed isotope source of fission fragments based on California-252, including a substrate with an active layer deposited on it and a sealing layer, characterized in that the active layer of California-252 is distributed over the entire surface of the aluminum substrate, and the sealing layer is aluminum oxide.  2. A method of manufacturing a sealed isotope source of fission fragments based on California-252, including the deposition of California-252 on the substrate and the formation of the sealing layer, characterized in that the deposition of California-252 is carried out by self-spraying in vacuum from the mother source by scanning on the entire surface of the aluminum substrate and the formation of a sealing layer of aluminum oxide is carried out by applying an aluminum layer over a layer of California-252, calcining an aluminum substrate with deposited layers When a temperature near the melting temperature of aluminum and subsequent anodic oxidation.