GB2118574A - Method and apparatus for the electrochemical development of damage tracks produced by nuclear particles - Google Patents

Abstract

This invention relates to a method and an apparatus for the electrochemical development of nuclear tracks in a sheet of insulating material 1 of which one face is covered by a thin metal layer 2. The method comprises the steps of subjecting the non metallized face of the irradiated sheet 1 to attack by a liquid electrolytic reagent 5 while the other face 2 is in contact with an insulating liquid 6. A constant or variable electric field is applied to the sheet so arranged by a voltage generator so that the metal layer is positively polarized. The electric field is applied to the sheet by respectively connecting an electrode 8 immersed into the electrolyte and the metal layer 2 to the generator poles. According to other embodiments, the metallized sheet face is not made to contact an insulating liquid but air. <IMAGE>

Description

SPECIFICATION Method and apparatus for the electrochemical development of damage tracks produced by nuclear particles The present invention relates to a method and apparatus for electrochemical development of nuclear tracks in insulating materials. Such tracks in addition to other damages can produce perforations of plastic sheets which may be far thicker than the lengths of the tracks.

Among the different systems for detecting charged heavy particles, the method of this invention is useful in the detection and dosimetry of fast neutrons, in criticality dosimetry in the detection of fissile impurities of low concentrations and of alpha emitters in biological and environmental samples, in the mesurement of radon emission when analyzing seismic faults in uranium prospecting, in the prospecting of high energy geothermal deposits, in industrial neturon radiography, in alpha autoradiography, in the production of biological and industrial filtering media, and in the evaluation of an insulating sheet resistance to electrical treeing which is a cause of electric cabie destruction.

Most of the applications of nuclear track detectors-whether nuclear emulsions or insulating transparent materials, chemically developed-are complicated by the fact that a high power microscope is required.

For counting the tracks produced by highly ionizing charged particles such as fission fragments the above difficulties have been overcome by the use of an automatic reader of holes called a spark counter. However such instruments can be used only with through holes, that is with holes which after a conventional chemical attack pass through the whole thickness of the detector, By such a technique an area of any extent can be counted in a fraction of a second while some ten thousands of microscope fields should be scanned to cover the same area.

Although the spark counter technique is already on the market in several countries such as U.S.A., Canada, Great Britain, Germany, Switzerland, its use is unfortunately limited to the counting of highly ionizing charged particles such as fission fragments. In fact the damage tracks produced by alpha particles and by the recoils produced by fast neturons in plastic sheets, are only some micron long after a chemical development that is not long enough for producing through holes in a plastic sheet of practical thickness which is d10 0 clam.

More recently, as a result of the invention of the electrochemical development pro cessdisclosed in Italian Patent No.

929,339 of which one of the inventors of the present invention is the author-the various applications of the detectors of tracks produced by low ionizing power particles such as fission fragments has been rendered practi- - cally feasible.

The electrochemical development method of the above patent makes it possible to develop nuclear tracks in solid insulating materials to such a degree that they become visible to the naked eye. In practice the electrochemical development of nuclear tracks in an irradiated detector sheet-that is in an irradiated insulating material sheets obtained by applying to the sheet an alternate electric field while the same sheet is subjected to a chemical attack adapted for developing the nuclear tracks already existing in their latent state in the sheet material.The nuclear tracks behave like needlelike faults which penetrate the insulating material and in their tips the electric field may be hundreds or thousands times higher that the average electric field that is such that various types of electrical phenomena are initiated thereby within the tracks (such as dielectric loss, electrical breakdown and so on). The nuclear tracks-initially some tens of Angstrom long--can be enlarged by the combined action of said electrical phenomena and chemical attack up to macroscopic dimensions such that they become visible to the naked eye. The count of the tracks can thus be made by sight.

Although a number of problems in the field of detection and dosimetry of alpha particles and neutrons have been solved by the electric chemical development technique so that it has been successfully applied in many laboratories all over the world such technique suffers from the following limitations: 1. insulating sheets much thicker than the track lengths are to be used, very high voltages ( > 800 volt rms) being consequently required; 2. the development process is drastically interrupted whenever a short circuit occurs due to a detector sheet perforation; 2. those tracks which penetrate the entire sheet thickness cannot be developed; 4. Automatic and then rather expensive--- counting systems are required; 5.The track superposition is high when the track density is higher than about 5000 tracks per square centimetre; 6. the development apparatus is complex.

The object of this invention is therefore to provide a method and an apparatus track for development such that the detector sheet can be of any thickness, such that the total thickness of the sheet can be penetrated by the track and such that voltages of only some tens of volt rms are required or even only constant voltages are required in the case of through tracks.

Another object of this invention is to avoid the short circuit occurring at each perforation of the insulating sheet and of transforming all the nuclear tracks--even if much shorter than the sheet thickness into perforations of the sheet so that they can be counted automatically by a spark counter.

Another object of this invention is that of providing a method and apparatus for the development of nuclear tracks such that the track superposition rate decreases with decreasing detector sheet thickness whereby high densities of tracks can be developed.

A further object of this invention is that of providing a method and apparatus such that a replica can be obtained of the track distribution in the insulating sheet with high contrast and resolution.

According to this invention a method and apparatus are provided for electrochemically developing nuclear tracks in plastic sheets in such a way that perforations are produced through said sheets with no short circuit occurring therefore.

According to the invention the plastic sheet functioning as a detector has one face covered or coated by a thin layer of metal: aluminum for example; said face is in contact with an insulating liquid while the non metallized face is in contact with an electrolytic reagent whereby only the latter face is developed.

The voltage to be applied for producing the electrochemical development can be either constant or variable while it can be variable when the tracks do not pass through the sheet.

In this invention, as with the invention of the patent previously cited, conducting paths are produced in the sheet by the chemical reagents used for the development. By applying to the sheet an appropriate voltage, either constant or variable, electric energy is deposited along the tracks-whether these are through tracks or not-and/or electric tracking phenomena are induced at the needlelike tips of the tracks which phenomena in combination with the chemical action of the electrolytic reagents are effective in producing macroscopic through holes in plastic sheets even if at the beginning the sheets are not totally perforated by the holes.

In a preferred method the invention, when a perforation occurs through the sheet, a circular area of metal much larger than the perforation hole is removed from the sheet by chemical, electrical and electrochemical effects and is replaced by. insulating liquid which causes an increase of the equivalent resistance of the through track when filled with electrolyte and concurs in eliminating the short circuit produced by the electrolyte passing through the entire thickness of the sheet.

According to another fet'jr Df the invention, when the voltage is applied across the sheet in such a way that the thin metal layer has a positive polarity-in those cases when a constant voltage is used for the track developmentor when the voltage although variable is applied in such a way that the metal layer has constantly a positive polarity, at each perforation corresponding to a nuclear track the metal is oxidized at the tracks or treeings whereby the equivalent resistance increases and the current flow is limited. Thus the electrical action is automatically reduced in the perforated track which is then subjected mainly to chemical action.In such a way the development of that particular track is remarkably slowed down while the reamining tracks can remain under the combined action of electrical and chemical phenomena until they also penetrate the entire sheet thickness.

It is another feature of the invention that the tracks are electrochemically developed to the extent of passing through the entire sheet thickness whereby the cross-sections of the tracks will be proportional to the sheet thickness, the later being then the determining element of the track superposition.

When the developing voltage is applied across the sheet in such a way that the thin metal layer has a negative polarity-in those cases when a constant voltage is used for the electrochemical development or when a variable voltage is applied to the metal layer so that the latter has constantly a negative polarity, the cathodic dissolution of the metal is very high, the current flow through the perforation is also very high and electric instability will often ensue with consequent destruction of the sheet. For such reasons the metal layer should always have a positive polarity.

With the invention the areas of metallized insulating material deprived of the metal layer represent a visible replica of the nuclear tracks.

According to the invention, a detector comprises a sheet of insulating material of any thickness of which one face is metallized, which sheet after being irradiated is placed between two cells of insulating material of which one contains an electrolyte which is also a reagent adapted for developing the tracks while the other contains an insulating liquid.

In a first embodiment of the invention, the apparatus comprises two prismatic cells each having an aperture on one of its sides and the sheet is clamped between the apertured sides of the cells. One cell is filled with electrolytic reagent and the other with insulating liquid.

The metallized face of the detector sheet connects with one terminal of a voltage generator whose other terminal connects with an electrode immersed in said electrolytic reagent.

In a second embodiment of the detector of this invention the use of insulating liquid is not required. According to such embodiment which will be called a detector package, it can simply comprise a metallized plastic sheet bonded to a plastic substratum relatively thicker than the sheet so as to function as a backing means therefor. Prior to bonding the sheet the metal layer is removed over the areas to be bonded to the substratum.

A number of detectors of this variant can be immersed into a tank where the chemical reagent is contained.

The electrochemical development can then be carried out simultaneously for all the detectors by applying the required voltage across the thin metal layer of each detector and the electrode sunk into the tank common to all of them.

In a third embodiment of the invention, the irradiated insulating material is partially metallized over a centre area of one face and is fluidtight coupled with an equal sheet with the interposition of a U shaped frame of insulating material; a boxlike cell is thus obtained which for the development is then filled with electrolytic reagent.

In a fourth embodiment of the invention two plastic sheets with one face metallized are bonded together along three of the four borders of the respective non metallized faces so as to form a plastic bag.

The detectors of the third and fourth embodiment can be developed by filling the boxes and bags with electrolytic reagent and applying the voltage across the electrolyte and the metal layer of their outer surfaces.

The present invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 shows a schematic vertical crosssection of an apparatus for electrochemical development according to the invention; Figure 2a shows an exploded view of a detector assembly, a "detector package", forming part of a second embodiment of the apparatus of this invention and adapted for the simultaneous development of a plurality of detector sheets; Figure 2b shows a perspective view of the elements of Fig. 2a coupled together; Figure 2b' shows an enlarged perspective view of a portion of Fig. 2b; Figure 2c shows a vertical cross-section of two detector packages conforming to Figs. 2a and 2b immersed in a tank filled with electrolytic reagent for electrochemical development;; Figure 3a shows an exploded view of a detector assembly, a "detector box", forming part of a third embodiment of the invention, adapted for the simultaneous development of a plurality of detector sheets; Figure 3b shows a perspective view of the elements of Fig. 3a coupled together; Figure 3c shows a vertical cross-section of three detector boxes conforming to Figs. 3a and 3b; Figure 3d shows an enlarged vertical crosssection of a detector box conforming to Figs.

3a and 3b; Figures 4a, 4b, 5a, 5b, 6a, 6b, and 7 show results obtained by the present invention and will be discussed hereinafter.

With reference to Fig. 1 the irradiated plastic sheet metallized on only one of its faces with a metal layer 2 is placed between two cells 3 and 4 of insulating material in which the chemical reagent 5 and the insulating liquid 6 are respectively contained; the insulating liquid being FREON in the illustrated case.

Cells 3 and 4 are provided each with an aperture 3'-usually circular-at the center of the cell wall facing towards the insulating material sheet the two apertures being aligned with each other but of different size such that the area of the aperture 4' of the cell wherein the insulating liquid 6 is contained is always larger than the aperture 3' of the cell 3 wherein the electrolytic liquid 5 is contained.

Any possible short circuit between the electrolytic liquid and the metallized surface is prevented by a tight adherence between the irradiated insulating material 1 and the two cells. The output of a constant or variable voltage is applied across the plastic sheet by means of an electrode 8 immersed into the electrolytic reagent 5 and the thin metal sheet 2 which is deposited on detector sheet 1 and which functions as the second electrode. The aperture of the FREON filled cell is larger than the reagent cell for ensuring the FREON action over the entire surface wetted by the reagent.

Gaskets 7 and 7' of insulating material are interposed with good mechanical engagement between cells 3, 4 and the irradiated insulating sheet 1 to function as a leakproof coupling when the two cells 3 and 4 are adequately forced one against the other.

The necessary voltage for the electromechanical attack is applied across the irradiated sheet 1 through the thin metal electrode 2 and electrode 8 immersed into the electrolytic reagent.

With reference to Fig. 2 a variant of the apparatus of this invention is shown therein.

As it appears from Fig. 2a such variant comprises an irradiated plastic sheet 1 coated with a thin metal layer 2 which may be in the form of a thin disc provided with an extension for connection to conductor 2' and with a substratum 1 3 which when bonded to the disc appears as shown in Fig. 2b and is called herein a "detector package".

A tank 14 filled with the electrolytic reagent is used for the electrochemical development, in which tank the detector packages 1 5 are immersed which in the example of Fig. 2c are only two but can be any number; the required voltage for the electrochemical development is applied to electrode 8 immersed into the liquid within the tank 1 4 and to electrodes 2' of packages 1 5. The constant and/or variable current is thus led to act through each of detectors 1 previously irradiated of which the damage tracks are to be developed.

With reference to Fig. 3 another variant is therein illustrated for the electrochemical development in which two detectors sheets 1 with the perspective metal layers 2 are bonded to either side of a support 1 6 having the form of a square frame lacking the upper bar so that when two sheets are bonded to the frame a small boxlike container is formed for holding therein the electrolyte liquid 5 adpated fro the development of nuclear tracks. In a particular cast not shown-said frame can be eliminated by fluidtight bonding the sheets to each other along their corresponding edges, the upper edge excepted.

Thus a bag is formed which functions as the above container.

Devices of this kind are shown in Fig. 3c in development arrangement; while only three of them are shown, they may be any number.

The required voltage is applied across the thin metal layers 2 of each pair of detectors--- which layers are on the outer faces of the container and the electrode immersed into the liquide reagent 5 within each container, all the devices being connected in parallel.

The operation of the apparatus of this invention is as follows: In the case of an apparatus comprising two cells as shown in Fig. 1, the irradiated insulating material 1 is interposed between cells 3 and 4 in such a way that the non metallized surface is in contact with the electrolytic reagent 5 adapted for the electrochemical development and the metallized surface is in contact with the insulating liquid 6; cells 3 and 4 are then filled respectively with reagent and with insulating liquid which in this example is FREON but may be another insulating liquid and then the output of a constant or variable voltage generator is connected across the thin metal layer and electrode 8 immersed into electrolytical reagent 5 for the time required to develop the tracks to such extent that holes are produced through the metallized sheets such that can be readily counted by means, for instance, of a spark counter.

In the case of the apparatus of Fig. 2 the plastic sheets are to be bonded to substratum 3 with the metal 2 facing the same; the devices so arranged are then immersed into tank 14 along with electrode 8. Substratum 1 3 serves for electrically insulating the metal electrode 2 from the electrolytic reagent 5 contained in tank 1 4. - A source of constant or variable voltage is then connected to electrode 8 and to the group of electrodes 2 consisting of the thin metal layers in such a way that the latter are constantly positive with respect to electrode 8.

Such connections are mal-tair,ed for the time as required for obtaining a perforation at each track.

Similar to the above is the use of the device of Fig. 3, that is the device in the form of a box or even in the form of a plastic bag. In the latter variant of the invention the device obtained by bonding two detector sheets to a frame or to one another is filled with electrolytic reagent 5 and the constant or variable voltage is applied respectively to the thin electrodes 2 over the outer faces of the sheet and to electrodes 8 immersed into each device, these being only three in Fig. 3c but could be any number. The voltage of the thin metal electrodes either in the case of a variable voltage or in the case of a constant voltage must be always positive with respect to electrode 8. The connection to the voltage generator is to be maintained for the time required to obtain a perforation at each track.

It is to be noted that the detector package of Fig. 2 and the detector box of Fig. 3-the latter obviously devoid of electrolytic reagent-lend themselves to being used as personal detectors of charged particles. After exposure to radiation they will be developed as per descriptions of the different variants of the invention.

Some examples of the results obtained by the development of tracks by means of this invention are illustrated in the figures following Fig. 3d.

The thickness of the insulating material to be used is not a limiting factor. In fact any thickness can be used. However the sheet handling is practically facilitated by the use of sheets thicker than 0,5 micron.

Figs. 4a and 4b illustrate two replicas obtained by irradiating two polycarbonate sheets 10 micron thick of which one face was covered by a thin aluminium layer-with the same fluence of fission fragments and subsequently developed electrochemically at ambient temperature in a solution of KOH 6N in water with FREON as insulating liquid and in a constant electric field of about 25 KV/cm which in this case corresponds to an applied voltage of only 1 5 volt.

In a case where the aluminium layer had a negative polarity, the electrochemical development had to be discontinued after 1h 30run because of the excessive current losses in the perforated tracks (Fig. 4a). With the aluminium layer at positive polarity (Fig. 4b) the electrochemical development was extended till 8 hours thanks to the excellent quenching action on the short circuit at each track due to the anodic oxidation of aluminium.

Figs. 5,5b illustrate a comparison of two replicas obtained by irradiating with fission fragments a polycarbonate sheet which in this instance was 40 micron thick and electrochemically developed at a variable voltage, the aluminium layer being alternately at a negative and positive polarity with respect to the electrode immersed into the electrolyte.

In the first case (Fig. 5a) a spark of propagatory kind occurred sometimes after a single perforation of the sheet which spark had a tendency to castastrophically destroy the sheet notwithstanding the FREON presence.

In the second case (Fig. 5b) the quenching of the short circuit at each perforation was excellent so that all the fission fragment tracks were developed and each produced a perforation.

Figs. 6a and 6b show replicas of samples identical to those of Fig. 5a,5b that is 40 micron thick which were both developed with the aluminium layer at positive polarity; but in one case (Fig. 6a) FREON was used while in the other case (Fig. 6b) no FREON was used.

It clearly appears from Figs. 6a and 6b how the effect of the insulating liquid FR EO N-in our case is that of producing circular areas consequent to the aluminium removal ,which are relatively smaller than those obtained without FREON.

Still according to the present invention, Fig.

7 shows a replica obtained with a cellulose triacetate 20 micron thick, aluminium coated on one face.After irradiation at contact with a sheet of natural uranium the detector sheet was developed electrochemically for three hours at ambient temperature by a solution of KOH 6N in distilled water,a positive polarity being applied to the aluminium layer ,the electric field being 25 KV/cm. at 2 KHz.

Fig. 7 shows a photomicrograph with aluminium areas removed at alpha tracks.

Claims (12)

1. Apparatus for the electrochemical development of tracks produced by nuclear particles in an insulating material sheet coated over one face with a thin metal layer, said development being obtained by electrochemically attacking said sheet with an electrochemical reagent and applying across the same an electric field, characterized in that during said development the metal layer is connected to one terminal of a voltage generator adapted for producing as desired constant or variable voltage while the other terminal of the same generator is connected to an electrode immersed into the electrolytic reagent.

2. Apparatus as claimed in claim 1, characterized in that two prismatic cells are provided, each with an aperture through one of its walls, that the irradiated insulating material metallized over one of its faces is fluidtightly clamped between said two cells in alignment with the respective apertures the cell towards which the metallized surface of the sheet is faced, being filled with an insulating liquid while the other cell is filled with an electrolytic reagent in which an electrode is immersed which is connected to one terminal of said generator while the other terminal is connected to the metal layer with which said face of the insulating material is coated.

3. Apparatus as claimed in claim 2 wherein said apertures are circular and coaxial, the one pertaining to the electrolyte cell being smaller than that pertaining to the insulating liquid.

4. Apparatus as claimed in claim 2 or 3, wherein the insulating liquid is FREON (RTM).

5. Apparatus as claimed in claim 1, characterized in that said metal layer with which the insulating material sheet is coated extends only over a limited central area of one of its faces and over a narrow strip from said center area to the upper edge of the sheet whereby the metal layer is connected to one terminal of the voltage generator through a wire; an insulating material plate being bonded to said insulating material sheet so that the metal layer is sandwiched between the sheet and said plate to form a detector package; said apparatus further comprising an insulating material tank in which any number of said packages are immersed; one of the generator terminals being connected to all the metal layers of said packages in parallel and the other terminal being connected to an electrode immersed into an electrolytic reagent contained in said tank.

6. Apparatus as claimed in claim 1, characterized in that said insulating material sheet is coated with a metal layer only over a central are of one face and said metal layer extends via a narrow strip to reach the upper edge of said sheet, the sheet being coupled with another like sheet with the interposition of an insulating material frame in the form of a U open upwards whereby a boxlike container is made; the non metallized surfaces of the two sheets facing one another and being fluidtight bonded to said frame; the metallized areas of the sheets being contained within the projection of said frames on a vertical plane; the space between the two sheets being filled with electrolytic reagent; an electrode being immersed in said reagent; all such said electrodes being connected in parallel to one terminal of a said voltage generator while the other terminal connects to all the metal layers of said boxlike containers in parallel.

7. Apparatus as claimed in claim 1 characterized in that one face of said insulating material sheet is coated with a metal layer over only a central area and said metal layer extends via a narrow strip to reach the upper edge of the sheet and that said sheet is twinned with another identical sheet fluidtightly bonded to the first one along its peripheral edge apart from the upper edge, whereby a bag is formed; the bonded edge not interfering with the metal layer; the non metallized surfaces of the two sheets facing each other; the gap being filled with electrolytic reagent during the development; an electrode being immersed in said reagent; and further characterized in that for the development any number of said bags are arranged in such a way that all the metal layers are connected in parallel to one of the terminals of said voltage generator and the electrodes immersed into the electrolyte of all the bags are connected in parallel to the other terminal of same generator.

8. Apparatus as claimed in any one of the preceding claims, wherein the metal layer with which one of the faces of the insulating material is coated comprises aluminium.

9. Method for the electrochemical development of tracks produced by nuclear particles in an insulating material sheet to the extent that each track penetrates the whole thickness of the sheet, which method comprises the following steps: exposing to the radiation to be detected an insulating material sheet coated at least partially on one face with a thin metal layer placing the irradiated sheet in a device in which the sheet face not coated with the metal layer is in contact with an electrolytic reagent into which an electrode is immersed while the other face is in contact with an insulating liquid; connecting the electrode immersed into the electrolyte to the negative terminal of a voltage generator; connecting the metal layer to the other generator terminal;; causing said generator to supply a constant voltage and maintaining said voltage for a time sufficient to develop the tracks present in the irradiated sheet.

1 0. Method for the electrochemical development of tracks produced by nuclear particles in a sheet of insulating material which method comprises the following steps: exposing to the radiation to be detected a sheet of insulating material coated at least partially on one face with a thin metal layer; placing the irradiated sheet in a device in which the sheet face not coated with metal layer is in contact with an electrolytic reagent in which an electrode is immersed while the other face of the sheet is in contact with an insulating liquid; connecting the electrode immersed into the electrolyte to a terminal of a variable voltage generator; connecting the metal layer to the other terminal of the generator;; causing the voltage generator to supply a variable voltage such that the metal layer is always positive with respect to the electrode immersed into the electrolyte and maintaining such voltage for a time sufficient to develop the tracks present in the irradiated sheet.

11. Apparatus for the electrochemical development of tracks produced by nuclear particles in an insulating material sheet coated over one face with a thin metal layer, substantially as hereinbefore described with reference to Fig. 1, Figs. 2a,b,b' an d c or Figs. 3a,b,c and d of the accompanying drawings.

1 2. A method for the electrochemical development of tracks produced by nuclear particles in an insulating material sheet coated over one face with a thin metal layer, substantially as herein before described with reference to Fig. 1, Figs. 2a,b,b' and c or Figs. 3a,b,c and d of the accompanying drawings.

1 3. A nuclear particle detector comprising a sheet of insulating material having a metal layer on one face, adapted for use in an apparatus as claimed in any one of claims 1 to 8 and 11, or a method as claimed in claim 9, 10 or

12.

1 4. The features hereinbefore disclosed or their equivalents in any novel combination.