DE2517866A1 - METHOD AND DEVICE FOR DETERMINING HEAVY IONS IN A BEAM - Google Patents

Description

Method and device for the determination of heavy ions in

a bundle of rays

The invention relates to a method and a device for determining heavy ions in a beam and to distinguish them on the basis of their specific energy losses when penetrating a dielectric Movie.

With this method and this device, breakthroughs are produced caused by charged particles in thin layers of solid dielectric.

Most known systems of this type have. A limited one Effectiveness when used to detect heavy ions. So in scintillation detectors there is a decline

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the. Scintillation effectiveness, if the * specific The energy loss of the charged particles increases.

The semiconductor detectors provide better energy but they cannot be made thin enough to hold heavy ions with energies of a few McV / nucleon promote and secure dE / dX performance characteristics.

In dielectric gas detectors, the energy losses of heavy ions through the detector windows are serious Obstacle.

In addition, the known detector systems are bulky and expensive, and they require high voltages and considerable equipment for signal processing.

There have been many new proposals over the past decade for the detection of heavy ions through the development of trace detectors (damage track detectors) have been made in insulating materials.

These detectors are very simple, space-saving and unexpensive. Their most important characteristic is their ability to differentiate from ionizing particles due to specific energy losses that are less than a certain threshold value.

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These detectors can be used for photography as well as rait Emulsions an occurrence of heavy ions can be localized and made visible. Unfortunately relate Information obtained using trace detectors and photographic emulsions on past and not on instantaneous events.

Furthermore, serious difficulties arise when using the known detectors when an automatic counting of the Traces is required.

As is known, the above-mentioned problems can be partially solved with the help of trace detectors by a special Type of spark counting detector is used. Such a detector contains an insulating film that is thin enough is that the traces can pass through the full thickness of the film. Subsequent chemical etching will be creates microscopic holes in the dielectric film. Two electrodes are then placed on the side of the film, which is placed on a plate made of plastic material.

When a voltage is applied to the electrodes, a breakdown occurs with sparking through one of these holes, which, due to the evaporation of the metal, creates a much larger hole in the thin aluminum electrode at this point will. This creates a single punch at each hole, so the total number of holes goes through

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simply counting the number of copies can be determined can.

These well-known facts have inspired the inventors to to develop a heavy ion detector analogous to the well-known Geiger-Müller counter with a spark chamber, but with a solid dielectric in which breakdowns are caused by charged particles.

Breakdowns caused by charged particles in dielectric Gas are well-known phenomena. About studies of such phenomena in solid dielectrics however, nothing has so far been reported.

Usually it takes too high voltages to breakdown in solid dielectrics and a single breakdown causes a short circuit in the test piece. Such high voltages and their destructive consequences can be caused by the The use of very thin dielectric films should be avoided.

The short circuits can be avoided by using the technique of the lane detectors described above; H. by Use of a capacitor, one electrode of which is very thin. This way, with every breakthrough, there will be a hole generated by the dielectric and by the thin electrode, the circuit being interrupted between the two electrodes will.

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With this technology, up to several tens of thousands

2
beats per cm.

However, various problems arise in practice. For example, a short circuit of the capacitor as a result of a dielectric Breakdown occurs at the connection point between the thin electrode and the feed line. this can be avoided by reinforcing the dielectric in the contact area of the line; in the same area can the Thickness of the thin electrode can be increased for the purpose of making better dielectric contact.

Furthermore, solid dielectrics always contain weak points where the breakdown area can be several times smaller than the extensive breakdown area of the dielectric. In capacitors with breakdowns that do not cause a short circuit, the weak points can be eliminated by _: Allowing the voltage on the dielectric film to gradually increase to the breakdown voltage of the ground.

The invention is therefore based on the object of creating a method and a device for the detection of heavy ions, where capacitors with a very thin solid dielectric are used.

Another object of the invention is to provide a method and a device for the detection of heavy ions

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create, whereby at the same time charged particles should be differentiated on the basis of their specific energy losses, d. H. those charged particles are to be detected, whose specific energy losses are greater than a predetermined threshold value.

Not only one event should be proven at the same time can be - that means the process of one, through a heavy ion caused breakdown - but also the automatic counting of the heavy ion, the localization individual events and the visualization of several events at the same time with the determination of a radiation pattern become possible.

Finally, with the help of the new method and the new device, only those ions whose specific Energy losses higher than a given threshold value can be detected, the threshold value being a function of the created field.

According to the invention, a method and a device for the detection of heavy ions is provided in which capacitors are used whose dielectric consists of a film whose thickness is in the range between 100 S and 10,000 S with at least one of the electrodes being very thin.

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The strength of this electrode must be so great that there is no breakdown can produce a short circuit in the dielectric and that the energy losses of the heavy ions within the electrode are negligible.

The invention takes advantage of the fact that in dielectric Films that are free of weak spots and that are subjected to an electric field and irradiated with heavy ions , breakdowns occur at values of the electric field that are lower than the dielectric strength of the Filrnniatcrials are.

Furthermore, the invention uses the principle that the higher the specific energy loss of the charged, the particle moving through the dielectric film, the lower the electric Fcld value, above which a Breakthrough occurs.

Heavy ions cause breakdowns in these capacitors only if a suitable voltage is applied to the two electrodes at the time of irradiation.

According to the invention, the capacitor is provided with a protective circuit to prevent a breakdown leading to permanent ionization connected to a voltage source. This is done through the use of a circuit for. Interruption the capacitor voltage at the start of the breakdown

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accomplished to its end. This type of circuit is used with Geiger counters.

A similar result can be achieved with the help of one in the feed circuit switched resistance can be achieved, whereby the charging of the capacitor is delayed.

According to the invention, a pulse counter is used to count the carbon copies connected to the capacitor. As the number of breakdowns increases, the reactance of the capacitor decreases corresponding to the reduction in the area exposed to the irradiation; accordingly, a correction of the heavy ion count is made necessary. Such a correction can be made simply by measuring the reactance loss of the capacitor happen. The holes that were created by the breakdown in the dielectric and in the thin electrode can be mediated an optical microscope can be observed and counted.

The invention is illustrated graphically in the accompanying figures. Show it

Fig. 1 is a plan view of a flat capacitor according to the invention,

Fig. 2 shows a cross section along the line II-II of the capacitor of Fig. 1 and the circuit diagram of the associated circuits,

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3 in the upper part shows the number of breakdowns produced by the crack fragments of Cf "" as a function of the applied voltage and in the lower part the derivation of this function,

Figure 4 is a photomicrograph of the breakdowns in a capacitor according to the invention,

Fig. 5 shows the number of breakdown points per cm of the capacitor surface in a device according to the invention, applied as a function of the number of split fragments, which penetrated the capacitor at a voltage of 29.5 V.

With reference to Figures 1 and 2, the device consists of the following parts:

A dielectric film 1, the thickness of which is in the range of 100 to 10,000, has a rectangular shape; he can but also be shaped differently. On its upper side, the dielectric film 1 is provided with a rib 4 running along it one of the sides of the rectangle extends and is close to this side. The rib 4 can by thickening the dielectric Film or by joining a strip of the same or different material to the film. Their task will be explained later. These dielectric films can be produced by vapor deposition or thermal oxidation

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Anodizing or by other known methods.

A very thin electrode 2, the thickness of which extends over the entire length of the gbcrfli before it must be kept constant is such that any breakdowns produced by heavy ions do not cause a short circuit and that the energy losses through the The passage of the charged particles through the electrode can be neglected.

As can be seen from Fig. 2, the electrode 2 has such a shape that the dielectric film to which it is applied, completely ναι her is covered. In the area of the rib 4, a thickening 5 of the electrode 2 is also provided, on the upper side of which there is a Contact 6 is located for connecting the electrode 2 to a voltage source, which will be described later.

The contact 6 in the example of FIG. 2 contains a pin semicircular end, the diameter of which is not greater than 0.5 mm.

This contact can be made by connecting the feed line to the electrode using supersonic or metallurgical technology Paths are made. The reason for the thickening 4 of the dielectric in the area of the contact 6 is, breakdowns in avoid this area.

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The device also includes a second electrode 3 of FIG Any thickness, which can be ordered from a conductive or semiconductive material and which, along with the thin electrode and with a dielectric, forms a capacitor. By means of a voltage source 8 is a controllable electrical Field applied to the electrodes.

A protective device 7 serves to limit or even switch off the voltage source from the electrodes at the beginning every breakdown and at least for the entire duration of the breakdown in order to prevent permanent ionization comes. Such a protective device can only consist of a resistance of at least 10 kOhm or alternatively As already mentioned - consist of a circuit which, at the start of the breakdown, is the voltage source of the capacitor switches off for a period of time that is equal to or greater than the duration of the breakdown.

A pulse counter or pulse frequency divider 9 is with the Capacitor connected for the purpose of counting the electrical impulses generated by the breakdown in the condenser.

In the device described, the thickening 4 of the dielectric can be omitted if there are only a few weak ones Place in the dielectric film.

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However, if there is such a thickening, it must with a gradual. Slope are executed so that sharp discontinuities in the electrical field are avoided \ earth.

Before being used for the detection of heavy ions, the device according to the invention is subjected to a method which has the purpose of eliminating any weak spots from the dielectric. To this end, it is gradually increasing Voltage is applied to the capacitor via the protective circuit 7 until a value of the electric field is reached that corresponds to the dielectric strength is comparable.

Then, when the dielectric film 1 is subsequently irradiated with heavy ions via the thin electrode 2, breakdowns occur at values of the electrical field which are lower than the field caused by the breakdown - d. H. the dielectric strength - the dielectric material. Every heavy ion generates a voltage surge that is caused by the pulse counter or pulse frequency divider 9 can be registered. Breakdowns are only made by those heavy ions generated whose specific energy losses in the dielectric are greater than a given threshold value. Such a threshold decreases as the applied electric field increases.

By visual inspection of the holes caused by punctures in the dielectric film 1 and the thin electrode 2

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their surface density can be determined in the various areas of the capacitor.

Some examples of test results obtained when using of the method and apparatus according to the invention for Detection of heavy ions have been achieved are shown in Fig. 3, 4 and 5 shown.

As already mentioned, the upper part of FIG. 3 shows the number of breakdowns produced by crack fragments from a Cf "" source, depending on the one at the

ο capacitors applied voltage, the surface of which was 0.1 cm ", wherein the dielectric consists of a SiO consisted ₀ -FiIm 1000 a, which was thermally grown on a ρ degenerate silicone film.

A SiO reinforcement of 700 Å * was used to reinforce the dielectric used in the contact area. The thin electrode consisted of an aluminum layer 300 Å * thick. The average Diameter of the evaporation holes in the aluminum electrode was about 20 microns when negative polarity was applied to the aluminum electrode. The voltage value at which spontaneous breakdowns occurred d. H. Breakdowns not caused by heavy ions was 70 volts.

As can be seen from Fig. 3, a plateau begins for the

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Number of breakdowns at about GO volts. In the lower part of FIG. 3, the derivation of the upper curve is shown as a function of the applied voltage, which is a hatred for the spectral distribution of the specific energy losses of the two types of crack fragments of Cf """ in a film of SiO ₀ of 1000 S represents thickness.

The values indicate. that the solution of the given problem can be obtained as a function of the specific energy loss of the charged particles in a solid dielectric film. FIG. 4 is a photomicrograph of the breakdowns produced by _{crack fragments in a SiO 0} film of 300λ thickness that thermally degenerated on a

+ 2

ρ silicon substrate was grown in a capacitor with 0.1 cm "surface. The thickening of the dielectric consisted of a SiO layer Voo S thick, while the thin electrode consisted of an aluminum layer 150 ″ thick. The density of the evaporated areas is in of the order of 5 χ ΙΟ ¹ "

per cm "corresponding to heavy ion breakdowns caused by plant a voltage of 29.5 volts were generated across the capacitor.

Fig. 5 is a graph of the penetrations per cm " a capacitor surface, plotted against the number of gap fragments per cm, which the capacitor with an applied Voltage of 29.5 volts crossed.

As the capacitor surface and thus its reactance

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i decreases as the number of breakdowns increases ; the determined number taking into account the loss corrected exposed surface.

As can be seen from Fig. 5, a linear course can easily

! the curve of the penetration number as a function of the number

'β 2

{of the split fragments up to 10 penetrations per cm of the

! Capacitor can be achieved.

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Claims (1)

Claims 1,} Method for determining heavy ions in a beam to distinguish them on the basis of their specific energy losses when penetrating a dielectric F ¹ Um, characterized by the following process steps: a) a bundle of rays is directed onto a device consisting of a thin metal layer, a layer of conductive Material and a film of dielectric material, to which the two mentioned layers are applied laterally, the thin metal layer being hit first by the beam, b) an electrical fold is applied to the layer, which is lower than the dielectric strength of the dielectric Material and is such that those heavy ions 509847/0782 whose specific energy losses in the dielectric material are greater than a predetermined threshold value, one in each case Cause breakdown between the layers mentioned, whereby the electrical fold has a protective device to reduce it the voltage is carried during the breakdowns, c) the breakdowns caused by the erions counted by means of a pulse counter connected to the device. d) Use an optical microscope to pinpoint the holes that box the punctures through the thin metal layer and the film made of dielectric material, visually inspected, with the average permeability per unit of the total exposed area and the unit average of the different areas of interest of the same surface to be determined. 2. The method according to claim 1, characterized, that before the process steps mentioned in claim 1, a preparatory process step is carried out by using the Protection device to the layers a voltage is applied, which gradually up to a value equal to the specific Dielectric strength of the dielectric material increased eliminates all vulnerabilities, i. H. the places where a Durjch-I shock could arise without being caused by a heavy ion, 509847/0782 ff ■ , be eliminated. 3. The method according to claim 1, characterized by a process step for correcting eggs counted on the exposed surface and ciu.f parts of the same Durc.hsch.la5e taking into account the Dlindv / ider- j standing losses of the device as a result of the previous Breakdowns occur, the correction being made by measuring the reactance loss of the device » 4. Device for the determination of heavy ions in one Beams of rays and to distinguish them on the basis of their specific energy losses when penetrating a dielectric film, characterized, that a thin metal layer (2) with a very thin dielectric Film (1) and a thicker layer (3) of conductive or semiconducting material so connected to one another are that they form a capacitor; that a controllable voltage source (8) with the capacitor (1, 2, 3) is connected to apply such an electric field to them that breakdowns in the capacitor only through those heavy ions are caused whose specific energy loss in the dielectric film (l) is a predetermined one Exceed the threshold value, that a protective device (7) table the voltage source (8) and the capacitor (1, 2, 3) is connected, which is suitable 509847/0782 is applied to the capacitor (1, 2,3) during one through one j Sehwerion caused a breakthrough applied voltage reduce such a value that the breakthrough does not j leads to permanent ionization (does not hecome propagative), and that an electrical pulse counter (9) for counting the electrical breakdowns caused by heavy ions in the dielectric of the capacitor (J, 2. o) were caused with the capacitor connected, the thickness of the thin metal layer ι (2) is chosen so that whenever a breakdown occurs through a Heavy ion penetrating the capacitor arises, thereby not causing a short circuit and the specific energy loss of the heavy ion penetrating through the dielectric can be neglected. 5. Apparatus according to claim 4, characterized, that the thickness of the dielectric film (l) varies between 100 Λ and 10,000 % . Z. device according to claim 4,
characterized, that the dielectric film (1) in the area (4) of the voltage contact (G) with the thin metal layer (2) has a gradual Has reinforcement. 509847/0782 7. Apparatus according to claim 6,
characterized, that the reinforcement (4) consists of a layer of a different material than the rest of the dielectric film (1). 8. Apparatus according to claim 4,
characterized, that the thin metal layer (2) in the area (5) of the voltage contact (6) is reinforced with the metal layer. 9. Apparatus according to claim 8,
characterized, that the reinforcement (5) of the thin metal layer (2) consists of a different material than the rest of the metal layer. 10. Apparatus according to claim 4,
characterized, that the protective device (7) consists of a resistance of not less than 10 kOhm. 11. The device according to claim 4,
characterized, that the protective device (7) contains a circuit which is capable of being connected to the capacitor (1, 2, 3) at the beginning remove applied voltage from each breakdown for a time interval greater than the duration of the breakdown is in the capacitor. 509847/0782 Blank page