GB2118760A - Method of making radioactive sources - Google Patents

Abstract

In the manufacture of radioactive sources, a compromise is reached between radioactive emission efficiency and physical containment of the radionuclide in the surface of a substrate. The method of the invention involves directing a laser beam at the substrate so as to cause the surface to melt locally and absorb an adjacent radioactive material thereon, and then removing the laser beam to effect rapid cooling and solidification. The substrate is preferably of metal, and may be caused to solidify in a micro- crystalline or amorphous state. Examples include americium-241, iron-55 and nickel-63 on stainless steel and copper substrates.

Description

SPECIFICATION Method of making radioactive sources This invention relates to radioactive sources comprising a radioactive material carried on or in a substrate. For efficient emission, it is necessary that the radioactive material should be on, or very close to, the surface of the substrate. But it is also necessary that the radioactive material should be bonded to the substrate in such a way that the radioactivity cannot readily be removed by corrosion or abrasion.

By way of illustration the majority of the alpha sources for ionisation chamber smoke detectors are made from rolled foil. In its production, alpha emitting nuclides are uniformly dispersed in a precious metal matrix which is encased and forged, also in precious metals, and then extended by power rolling.

In this latter process the face covering the active material is reduced to a thickness which is chosen as a compromise between emission efficiency and nuclide containment. Subdivision of foil so produced provides small foil pieces in the form of disc, square or strip. The process is labour intensive.

It is an object of the present invention to provide a method for the manufacture of radioactive sources having improved emission efficiency without loss of nuclide containment. In such sources the radioactive material may be distributed in a micro-crystalline or amorphous surface layer of a metallic substrate.

The invention provides a method of making a radioactive source, which method comprises providing a substrate, preferably a metallic substrate, and a radioactive (or potentially radioactive) material adjacent a surface thereof, applying an intense source of energy to the said surface so as to cause a surface layer of substrate to melt and absorb radioactive material, and then removing the source of energy, whereby the surface layer rapidly cools and solidifies with the radioactive material distributed therein.

The surface layer of a wide range of metallic substrates, if cooled sufficiently rapidly, can solidify in the form of a micro-crystalline or amorphous material. The distinction between micro-crystaliine or amorphous metals and ordinary crystalline metals is well reported. By "micro-crystalline or amorphous metal" we mean a metal in which crystals are either absent or else have a size which is substantially smaller than the crystal size of a metal of the same composition produced conventionally.

It may sometimes be convenient to incorporate in the substrate a material which is not itself radioactive but which is intended to be made radioactive, e.g. by irradiation. Such materials are included within the term "radioactive material" as used herein.

The substrate is a material whose surface can be melted to a desired depth and then rapidly cooled to yield on solidification a surface retaining the deposited material. The substrate will generally be a metal. Different metals will be chosen for different purposes, bearing in mind the physical and chemical conditions to which they will be subjected in use. For cr-sources for smoke detectors, useful metals include titanium and, particularly, stainless steel. The substrate will generally be massive, e.g. a sheet or strip of metal on which isolated deposits of radioactive material are formed and heated as described above in local areas so that the bulk of said sheet or strip forms a heat sink.

The radioactive material is preferably an emitter, a low-energy emitter or a low-energy yemitter; with such nuclides, very thin coverings are required if efficient radiation emission is to be achieved.

The radioactive material may be provided in various physical and chemical forms. For example, a radioactive gas atmosphere may be maintained adjacent the substrate. More preferably, the radioactive material may be deposited in solid form on the substrate. The chemical form of the material deposited is not of great importance, provided that the deposit is sufficiently non-volatile to remain on the substrate when the surface thereof is melted. The deposit may be metallic or non-metallic in nature. The following are examples of materials which may be used:- Americium-241 as the oxide.

Nickel-63 as the metal or oxide.

Carbon-14 as methane or carbon dioxide or the element or a metal carbide.

Cobalt-57 as the metal or oxide.

lron-55 as the metal or oxide.

Gadolinium-i 53 as the oxide.

Cadmium as sulphide.

A metallic material may be deposited by any convenient technique, such as electrolytic deposition, vacuum evaporation, sputtering or ion implantation. There should be intimate metallic bonding to the substrate. It is believed that good binding facilitates subsequent entry of the deposited material into the melt.

A non-metallic material may be deposited by any convenient technique; such as in aqueous solution. Again, intimate contact between the deposit and the substrate is advantageous. It is therefore preferred to promote bonding of a non-metallic material to the substrate by a preliminary heat treatment which does not melt the substrate. For example, a stainless steel substrate carrying a nonmetallic deposit may suitably be heated to from 7000C to 8000C.

To melt the surface of the substrate to a desired depth, there is used a source of intense heat energy, such as for example an electron beam, a spark discharge, or, more particularly, a laser. The technique of laser glazing is well known, though it has not previously been used to introduce radioactive material into a substrate. The technique of laser glazing involves applying to the surface of the substrate a laser beam focused to a power density in the range 104 to 108W/cm2, preferably 5 x 104 to 5 x 106 W/cm2. The procedure yields a thin molten surface layer on a substrate which is relatively cold and remains solid. Due to the steep temperature gradient established by the process, rapid solidification and subsequent solid-state cooling takes place following the removal of the laser beam.Average quench rates in excess of 1 080 C/second have been achieved in melt thicknesses in the 1 to 10 micron range, with correspondingly lower quench rates in thicker melt sections. The relationship between laser power density interaction time and melt depth may be calculated by finite-element analysis procedures for materials with accurately known thermal properties. Typical interaction times for the purposes of this invention lie in the range 10- to 10-7 seconds, particularly 10-1 to 10-3 seconds.

It is convenient to work with a pulsed laser, for example a noedymium-yttrium-aluminium-garnet laser providing 1 to 30 joules, preferably 5 to 10 joules per pulse and a few pulses per second, the substrate being moved relative to the beam between each pulse. Alternatively, a continuous laser such as a carbon dioxide laser may be used, in which case the substrate is continuously or intermittently traversed through the laser beam.

The optimum depth of surface melting, and hence of diffusion of the radioactive or other deposit, depends on the nature of the deposit and is a compromise between source efficiency and containment.

For americium-241, a suitable melt depth is 2 microns. The melt depth is controlled by varying the power density absorbed by the substrate and the time for which it is absorbed.

Upon removal of the source of energy, the molten surface layer is rapidly cooled through its melting point and down to ambient temperature. That maximum rate of cooling should be at least 1050 C/second and preferably at least 1 C/second. This rapid cooling may cause a metallic substrate to solidify as very fine crystals, or even in amorphous form. It is known that lazer-glazing techniques can substantially improve the hardness and wear-resistance properties of a surface.

-The following Examples illustrate the invention. Example 2 differs from Example 1 in the provision of the deposit on the substrate prior to treatment with the laser.

Example 1 AlSl3 1 6L stainless steel strip was "dimpled" and americium-241 nitrate solution dispensed in approximately 0.8 ,uCi quantities. After evapbration of solution, the strip was heated to 450-5500C in air.

The active areas of the strip were then laser glazed using a neodymium-yttrium-aluminium-garnét laser. The complete area of the deposit was glazed by means of overlapping spots arranged in a spiral pattern. The spots were 0.9 mm in diameter and were generated by a pulse energy of 5 joules art a pulse length of 10 m sec. Pulses were at the rate of 2 per second, and the substrate was moved between each pulse.

ez-Spectra of the sources indicate that the americium-241 has been incorporated into the metal surface. The principal energy of americium-241 is 5.48 MeV, and material remaining on the surface of the substrate would produce an emission at this energy. The a-spectrum of the glazed sources shows the most abundant emission at an energy 5.37 MeV. The peak width is typically (FWHH)0.-1-MeV.

There is virtually no activity at 5.48 MeV, indicating that virtually all the americium-241 is below the surface of the substrate. The peak energy of 5.37 MeV compares favourably with the peak energy of conventional foil sources containing americium-241 which typically have a peak energy of about 4.5 MeV.

In all the sources, the substrate had a microcrystalline surface layer in which the radionuclide was distributed.

The sources were subjected to integrity testing, and the results are set out in the Table below.

A wipe test (Test A to BS 5288/APP D 2.1) to assess removable activity.

An immersion test (Test (L) to BS 5288/APP D 2.3) involved immersing the source in water for four hours at 500C.

For the U.S. Underwriters (SO2) test, the source was contained in an enciosure over 100 cc of water and was subject to a daily influx of sulphur dioxide and carbon dioxide for 10 days. Subsequently it was wipe tested and overnight immersion tested.

The Underwriters H2S test is similar to the SO2 test, except that hydrogen sulphide replaces the sulphur dioxide and carbon dioxide.

The BS3 16 6 corrosion test involves subjecting the source to 1 6 days exposure to a sulphur dioxide environment.

EXAMPLE 2 There were used for this experiment two Am-241 sources, of a kind which are commercially available, having an activity of 1.5 microcuries. The sources had been formed by vacuum sublimation of Americium-241 on to a lightly oxidized stainless steel substrate, followed by firing the substrate at 8000C for 1 minute in air. The sources were subjected to laser glazing under the same conditions as Example 1 and then to integrity testing as described in Example 1, with results as set out in the following Table.

Integrity Testing Results following Laser Glazing

Example 1 Example 2 source source Test 1 2 3 4 A B ~ ~ 1. Measured Activity yCi 0.82 0.73 0.61 0.89 1.5 1.5 2. Wipe Test A (nCi) 0.65 2.4 0.97 1.79 0.07 0.08 3. Immersion Test L (nCi) 0.88 0.45 07 0.91 0.60 0.42 4. Peak a energy MeV 5.37 5.37 5.38 5.37 5. Underwriters (SO2) Test Wipe Test A (nCi) 0.03 0.88 Overnight immersion (nCi) 0.75 0.61 (nO I) 0.75 0.61 6. Underwriters H2 S Test Wipe Test A (nCi) 0.75 1.84 Overnight immersion (nCi) 1.38 5.6 7. BS3116 Corrosion Te'st Wipe Test A (nCi) 1.09 - 0.64 Overnight immersion (nci) 58 0.17 ' (nCi) ' 2!58 0.1J 8. .Salt Spray Tesf (5 days) (5 days) Wipe Test A (nCi) 0.23 Overnight immersion (nCi) 0.21 lon currents were measured before aid after each test. N6 change was detected for any of the sources.

After having been subjected to the testing set out in the above table, the sources of Example 2 were further subjected to the testing procedure given in 1 So.29 19 and BS.5288. Both sources achieved the rating C54242. The further tests passed were: a) Temperature from --400C (20 minutes) to +6000C (1 hour) and thermal shock 6000C td 200C.

b) External pressure from 25kPa absolute to 7MPa absolute.

c) Impact, 50 g from 1 m.

d) Vibrations, 90 minutes 25 Hz to 80 Hz at 1.5 mm amplitude peak to peak, and 80 Hz to 2000 Hz at 20 g,.' e) Puncture, 1 gfrom 1 m.

EXAMPLE 3 In a further sequence of trials, molybdenum sheet 1.0 mm thick surrounded by an atmosphere of C-i 4 methane at approximately atmospheric pressure and of specific activity 3 mCi/mmol was housed in a glass vessel possessing a suitable port to facilitate laser beam entry.

Repeated pulses of IOJ at a pulse length of 10 msec and repitition rate of 5 Hz was used to produce a glazed area on the molybdenum surface consisting of adjacent individual spots.

Subsequent measurement of the activity of the areas indicated levels in the region 12 ,uCi. One of the sources of activity was mounted in a smoke detector ionisation chamber which functioned satisfactorily in detecting smoke.

These sources satisfactorily passed statutory wipe and immersion testing.

EXAMPLE 4 Fe 55 as metal (300 microcuries) was electro deposited onto a copper substrate disc 12.5 mm diameter by 3.0 mm thick.

Laser treatment:-- Pulse energy 8J, pulse length 3 msec, pulse rate 7/sec, effective treatment area/pulse 0.25-0.3 mm diameter.

The complete active face (12.5 mm diameter) was treated at a scanning speed of 1.25 mm/sec.

The ratios of total activity removed in 3 wipe tests (a) before, (b) immediately after and (c) 1 week after glazing are compared to those from the control sample. Result (d) was taken 2 weeks after glazing.

Glazed (a) 1.0 (b) 0.37 (c) 0,06 (d) 0.065 Unglazed control (a) 1.0 (b) 0.559 (c) 1.552 EXAMPLE 5 Ni 63 as metal (300 microcuries) was electro deposited onto a copper substrate disc 12.5 mm diameter x 3 mm thick.

Treatment conditions were as Example 4.

The corresponding ratios were: Giazed (a) 1.0(b) 0.39 (c) 0.17 (d) 0.154 Unglazed control (a) 1.0 (b) 0.49 (c) 0.72 It is noted that in Examples 4 and 5 the removal of activity diminishes with successive test sequences following glazing compared to the absence of systematic reduction in the unglazed control.

Both sets of tests showed a reduction of 1 5-1 8% in the measured emission. Careful checking of the surround showed no evidence of removal of activity from the source to the containment indicating that the material has been incorporated with the substrate.

Further evidence is provided by the observation that prior to glazing the surfaces had a silvery colour due to the active deposits (Fe or Ni) but on completion of glazing the surface had reverted to a copper colour.

EXAMPLE 6 Fe 55 as oxide (300 microcuries) was precipitated onto a stainless steel disc 25 mm diameter x-0.25 mm thick as a 5 mm active area (i.e. active loading 1 mCi/cm2).

Laser Treatment.

Pulse energy 3.0J, pulse length 20 m sec., pulse frequency 7/sec, scanning speed 1-25 mm/sec, beam spot size 0.7-0.8 mm diameter.

The effect is described in terms of the fraction of activity removed by the standard wipe test, viz.

0.38% from the glazed sample compared to virtually complete removal from the untreated sample.

EXAMPLE 7 Ni 63 as oxide (200 microcuries) was precipitated onto a stainless steel disc 25 mm diameter x 0.25 mm thick as a 5 mm active area (i.e. active loading 1 mCi/cm2). Following similar laser action the removable activity was 1.07% compared to about 100% from the untreated control.

The samples of Examples 6 and 7 are of relatively high loading for several appiications; however, the results illustrate the effective immobilisation of 99.6% of the activity (Fe 55) and 99% for Ni 63.

The advantages of the method of this invention compared to the conventional foil production process arel a) the method can conveniently be operated on a continuous basis.

b) precious metals are not involved.

c) volumetric dispensing of precise amounts of nuclides per source is a relatively simpie matter, and corrections easily made.

d) in continuous production, source finishing, quality control, measurement and shaping can all be made part of the production line, thus avoiding the need to handle large numbers of small components.

e) sources can be presented, tested and measured in despatch batches.

f) the sources can be arranged to have a higher radition efficiency without loss of integrity.

Claims (9)

1. A method of making a radioactive source, which method comprises providing a substrate and a radioactive material adjacent a surface thereof, applying an intense source of energy to the said surface so as to cause a surface layer of substrate to melt and absorb radioactive material, and then removing the source of energy, whereby the surface layer rapidly cools and solidifies with the radioactive material distributed therein.

2. A method as claimed in claim 1, wherein the substrate is metallic.

3. A method as claimed in claim 2, wherein the surface layer of the metallic substrate cools and solidifies in the form of a micro-crystalline or amorphous material.

4. A method as claimed in any one of claims 1 to 3, wherein the radioactive material is an alphaemitter, a low-energy beta-emitter or a low-energy gamma-emitter.

5. A method as claimed in any one of claims 1 to 4, wherein a solid radioactive material is provided adjacent a surface of the substrate in a form sufficiently non-volatile to remain when the surface of the substrate is melted.

6. A method as claimed in claim 5, wherein the substrate carrying a deposit of a radioactive material thereon is subjected to a preliminary heat treatment under conditions which do not melt the substrate.

7. A method as claimed in any one of claims 1 to 6, wherein the intense source of energy applied is a laser beam.

8. A method as claimed in claim 7, wherein the laser is operated at a power density of 5 x 104 to 5 x 106 W/cm2 for an interaction time in the range 10-1 to 10-3 seconds, so as to generate a melt thickness of from 1 to 10 microns on the substrate surface.

9. A method as claimed in any one of claims 1 to 8, wherein the radioactive material is americium241.