Time Base Combining Radioactive Source And Solid-state Detector

Abstract

A radioactive timekeeping standard constituted by a radioactive source of alpha particles combined with a solid-state radiation detector, the source being in the form of a backing having a planar array of discrete islands of a radioactive isotope, the alpha particles emitted therefrom passing through an apertured mask and impinging on the sensitive surface of a solid-state radiation detector, the geometry of the mask apertures, which are in a matching array, being such as to restrict emanations impinging on the detector surface to substantially normal angles of incidence and in addition preventing particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island.

Description

Related Applications: (A) Ser. No. 651,864, filed July 7, 1967, now abandoned of Koehler and Grissom, entitled "Timepiece with Radioactive Timekeeping Standard" and (B) Ser. No. 700.102, filed Jan. 24, 1968, of Koehler, entitled "Multicellular Solid-State Radiation Detector Assembly."

This invention relates generally to radioactive time bases, and in particular to a timekeeping standard constituted by a radioactive source of alpha particles combined with a solid-state radiation detector.

In the above-identified application (A), there is disclosed a timepiece arrangement in which a radioactive source having a relatively long half-life emits charged particles which are sensed by a solid-state radiation detector. The detector yields a relatively large number of electrical pulses per second, the pulses being scaled down by electronic pulse frequency dividers to produce a low number of control pulses, such as one per second. The periodic control pulses are applied to an electronic or electromechanical time register to actuate or control the register to indicate time. The combination of the radioactive source and detector is designated a timekeeping standard, as distinguished from the associated pulse scaling and indicating stages.

Although nuclear disintegrations are distributed randomly in time, timing accuracy can be obtained through the accumulation of a sufficient number of counts. Since the disintegrations obey the Poisson distribution in accordance with probability theory, one can calculate the statistical accuracy that can be expected from a total number of counts, assuming that the counting system contributes negligible error.

As pointed out in copending application (A), the preferred form of radioactive source for the timekeeping standard is an isotope which emits alpha particles and has a prolonged half-life. While gamma rays are radiated with discrete energies, and in that respect are nearly monoenergetic and can be used for timing purposes, they are a highly penetrating form of radiation; hence it would not be feasible, within the confines of a watch or small timepiece, to provide the necessary protective shielding. Also, it would not be possible with gamma rays to control the area of the detector to be exposed to the radiation source.

Beta particles, on the other hand, are not emitted with discrete energies, but have a continuous distribution of energies. This radiation is a high-speed electron that is emitted at the transformation of a neutron to a proton within the nucleus of an atom. While it is possible to effect shielding of beta particles with a few millimeters of aluminum, timing control is very difficult since the particles are not monoenergetic.

The reason for this is that the output pulse heights of a solid-state radiation detector are proportional to the ionization produced by incident radiation. Each nuclear particle of the same type will lose approximately the same proportion of energy through the ionization process, thereby establishing a direct relationship between the pulse height of the detector signal and the energy of the radiation. Unless the radiation is nearly monoenergetic, electronic instabilities in the system can cause variations in the detection of low energy pulses and it becomes difficult to discriminate between detector output pulses and electrical noise inherent in solid-state detectors and associated amplifiers. This gives rise to undesirable variations in timekeeping.

Alpha particles consist of two protons and two neutrons, and possess a charge twice that of an electron but opposite in sign, as is also the case for a nucleus of a helium atom. The quantity of energy released is discrete, its magnitude being characteristic of the particular alpha particle-emitting radioisotope. Naturally radiated alpha particles have energies ranging from approximately 4 to 10 m.e.v. The fact that alpha radiation is highly ionizing accounts for its relatively short range when traversing matter. This range is only a few centimeters in standard air, and several sheets of ordinary paper will absorb even the most energetic of alpha particles. Yet from the characteristic properties of gamma, beta and alpha particles, it is evident that only alpha particles are suitable for radioactive timekeeping standards, for not only are they nearly monoenergetic, but they can be handled in a practical sense within the confines of a small timepiece.

When alpha particles are radiated from a relatively thick source, alpha particle energies are absorbed within the thickness of the radioisotope deposit itself. Thus a continuous distribution of energies will result from alpha particles being radiated from various depths in the thick layer. The spread of this distribution can be minimized by obtaining the required activity from the thinnest source possible.

Another cause of spread or departure from monoenergicity, is the airgap between the radioactive isotope and the detector. While this can theoretically be overcome by placing the detector and source in a vacuum, this solution is not practical. A more workable approach is to place the source in intimate contact with the detector. However, existing semiconductive solid-state detectors are physically constructed with a thin entrance window through which the particulate radiation must pass before entering the sensitive volume or depletion zone of the detector. Though one can make this window very thin, the intimate contact geometry results in varying degrees of energy degradation. At small angles of incidence, this degradation reaches a level equal to the total energy of the incident particles.

One solution to the entrance angle effect is to displace the source from the detector by a distance which is such as to admit only radiation whose angle of incidence is about normal to the detector surface. However, this remedy is not practical in a small timepiece because of physical size limitations.

In view of the foregoing, it is the primary object of my invention to provide a time base constituted by a radioactive source of alpha particles in combination with a solid-state radiation detector in an arrangement which minimizes the airgap therebetween and yet effectively restricts incident radiation to nearly normal angles of incidence, thus obviating a spread in energy distribution.

More specifically, it is an object of the invention to provide a timekeeping standard assembly of the above type in which an apertured mask or screen is interposed between the radioactive source of an alpha particle and the detector to so confine emanation from the source as to maintain it nearly monoenergetic.

Also an object of the invention is to provide an efficient and reliable assembly of radioactive source and solid-state radiation detector.

Briefly stated, these objects are accomplished in a radioactive time base assembly comprising a backing having an array of discrete islands thereon of a radioactive isotope emitting alpha particles and having a relatively protracted half-life, a mask being interposed between the islands and the surface of a solid-state radiation detector, the mask having a matching array of apertures therein whose geometry is such as to confine the particulate energy impinging on the detector surface to substantially normal angles of incidence and to prevent particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island.

For a better understanding of the invention, as well as other objects and further features thereof, reference is had to the following detailed description to be read in conjunction with the accompanying drawing, wherein:

FIG. 1 schematically illustrates an assembly composed of a single layer of radioactive material and a detector, an apertured mask being interposed therebetween, this illustration being for purposes of background analysis;

FIG. 2 schematically illustrates the behavior of the device shown in FIG. 1;

FIG. 3 schematically shows a radioactive timekeeping standard in accordance with the invention;

FIG. 4 illustrates the behavior of the standard shown in FIG. 3;

FIG. 5 is an exploded perspective view of an assembly of the type shown in FIG. 3; and

FIG. 6 is a modified form of standard in accordance with the invention.

RADIOISOTOPES

The requirements for an alpha-emitting radioactive isotope will now be considered. Although many radioisotopes with natural alpha radiation are commercially available, most of them are not suitable because their half-lives are not sufficiently protracted to satisfy the half-life requirements for a timekeeping standard as set forth in copending application (A). The following radioisotopes are considered suitable for timekeeping purposes, in addition to those already identified in said copending application: ##SPC1##

As shown in FIG. 1, a layer 10 of the selected radioisotope is formed on a backing 11, which may be of platinum or aluminum, or any other material providing adequate support and preferably having shielding properties. To minimize the spread of energy distribution, the layer is made as thin and as uniform as possible. To this end, a deposition technique may be employed, the radioactive material being laid down in a very dilute solution on the backing and then allowed to dry, the resultant film adhering to the backing.

Detector 12, which is used to intercept alpha particles emanating from layer 10, may be of the surface barrier or diffused-junction type commercially available. While the present invention resides in the use of an apertured mask in combination with a radioactive source in the form of an array of separate islands of radioisotope material, the mask 13 in FIG. 1 is shown in combination with a single, continuous radioactive layer 10, and is provided with an array of apertures 13A, 13B, 13C, etc., defining parallel passages of uniform cross section for the emanations. This combination is not in accordance with the invention, but is shown only for purposes of background analysis.

FIG. 2 indicates the trajectories of particles emanating at various angles from different points on source 10, and traveling toward the surface of detector 12. Path P.sub.1 is normal to the surface of detector 12. This is the shortest and most direct path and provides maximum energy. The angle of incidence of path P.sub.2 is such that it passes through the upper edge of the mask, some energy being absorbed therein, whereby the remaining energy of the particles arriving at the detector is reduced. Path P.sub.3, which cuts through the lower edge of the mask, is even further reduced in energy. Similarly, paths P.sub.4, P.sub.5 and P.sub.6 are intercepted by varying thickness of the solid body of the mask, and are more or less reduced in energy by absorption.

Thus the particles in path P.sub.1 will produce a relatively large output pulse in the detector, whereas those in the other paths will produce pulses having lesser and varying degrees of amplitude. Hence while the alpha-emitting source is nearly monoenergetic, the detector responds as if the source had a spread of energy distribution, which is undesirable for timekeeping purposes.

OPERATING PRINCIPLES OF THE TIMEKEEPING STANDARD

Referring now to FIGS. 3 and 4, there is schematically shown a timekeeping standard in accordance with the invention and comprising a backing 11 on which is deposited a planar array of thin islands 14A, 14B, 14C, etc., of a radioisotope emitting alpha particles. Interposed between this radioactive source and the exposed surface of the solid-state radiation detector 12 is a mask, generally designated by numeral 15, having an upper section I and a lower section II to form a matching array of apertures 15A, 15B, 15C, etc.

Each island 14A, 14B, etc. is centered with respect to the upper zone of the corresponding aperture defined by the upper section I, which upper zone has a relatively large and uniform cross section, the diameter of the island being equivalent to or less than that of the upper zone. The lower zone of the aperture defined by the lower section II, has at its top side a smaller diameter preferably equal to or greater than the diameter of the associated island, the underside of the aperture being chamfered to provide a flared mouth of increasing cross section.

The preferred geometry of the mask structure eliminates those events which would cause an energy loss in the aperture edge adjacent the radioactive island. Thus it will be seen that the trajectories indicated by emission paths Pa, Pb and Pc normal to the detector surface, are unobstructed by the mask. Paths Pd and Pe, which represent very low angles of incidence are intercepted by the upper section I of the mask and completely absorbed thereby.

Because of the flared lower edges of the apertures, paths Pf, Pg and Ph, which are not normal but which have relatively high angles of incidence, go directly to the detector surface without striking an aperture edge and hence without being degraded. Virtual elimination of the detector-side aperture edge by flaring causes a minimization of the solid angle subtended by absorptive mask material at the source, thus minimizing the number of particles that can actually be energy degraded in the mask material.

Thus the apertured mask in accordance with the invention prevents particles from any one island from impinging on a neighboring portion of the detector surface at a low angle of incidence, and provides an entrance aperture subtending an optimum angle at the detector. The geometry of the aperture in the mask is such as to minimize edge effects as well as to reduce airgap losses.

STRUCTURE OF TIMEKEEPING STANDARD

Referring now to FIG. 5, there is shown a practical embodiment of a timekeeping standard in accordance with the invention. Backing 11 for the radioactive source is in the form of a thin disc of suitable shielding material on which is deposited a uniform array of thin circular islands 14A, 14B, etc., of radioactive material possessing alpha-particle-emitting properties. The islands are constituted by the deposits of radioactive material substantially equispaced from each other.

Mask 13 includes a circular upper plate I having relatively large apertures in a configuration matching the array of the islands, the diameter of the plate being equal to that of the backing 11. Mask 13 is provided also with a lower plate II having a corresponding array of smaller apertures whose underside (not shown) is flared, as indicated in connection with FIG. 3. Finally, below plate II is a disc-shaped solid-state radiation detector 12.

When the four discs are brought together, the resultant wafer constitutes a highly compact and efficient timekeeping standard which may readily be incorporated in a small timepiece or watch. In a structure of this type, the geometry of the mask is such as to restrict emanations impinging on the detector surface to nearly normal angles of incidence and furthermore preventing particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island.

Preferably, the diameter of each island or the diameter of the circle circumscribing the island, should the island not be circular, is no greater than twice the distance between the surface of the island and the plane of the detector, the diameter of each mask aperture being not less than the diameter of the island or the circle.

MODIFIED FORM OF TIMEKEEPING STANDARD

In the conventional solid-state radiation detector, an electric field is set up across a low-conductivity region, which region is the charge depletion layer at the diode junction operating at reverse bias. When a charged particle passes through the semiconductive medium, electron hole pairs are produced therein. These charges are caused to separate by the electric field and the resultant electrical signal can be transmitted to a measuring system to afford useful information respecting the particles detected.

The principal drawback in existing solid-state detectors is that its sensitivity, especially to low-energy particles, tends to be very low, for there is an appreciable probability of absorption of such particles before they reach the depletion layer, and even if a pair of charges is produced in the depletion layer, the quantum efficiency is limited to one pair per particle, with no chance of multiplication such as is effectively obtained in Geiger-Muller tubes and proportional counters.

The low sensitivity dictates the use of high-gain amplifiers. Thus in the case of detectors 12 shown in the previous figures high-gain amplication is necessary. However, the output signal from a conventional solid-state radiation detector lies in the millivolt range and is not much more pronounced in amplitude than the noise level in the associated electronic amplifying circuits for elevating the signal to a level suitable for measurement and analysis. This noise may give rise to spurious signals which cannot readily be distinguished from the radiation signals, thus adversely affecting the sensitivity and energy resolution of the detection system.

In my copending application (B), there is disclosed a multicellular, solid-state radiation detector assembly adapted to produce exceptionally large signals in response to incident radiation, the detector being constituted by an array of individual surface-barrier or diffused-junction, radiation-sensitive, semiconductive cells, each of which has a small area and a low internal capacitance.

The cells in the array are unidirectionally connected in parallel relation with respect to current flow, but are otherwise electrically isolated from each other, whereby the overall capacitance of the array is low while the detection efficiency thereof is substantially equal to a unitary radiation detector whose surface area is equivalent to the aggregate area of the cells, the signal output from the multicellular detector being far greater than that yielded by the unitary detector.

In the arrangement shown in FIG. 6, the multicellular solid-state radiation detector is combined with an array of radioactive islands 14A, 14B, etc., and an apertured mask 13 of the type shown in FIG. 3. The multicellular detector is constituted by an array of tiny radiation detector cells 16A, 16B, 16C, 16D, etc., whose diameters are substantially the same as that of the radioactive islands and which are positioned in registration therewith.

Cells 16A, 16B, etc., are unidirectionally connected in parallel relation with respect to current flow by diodes 17A, 17B, 17C, etc., but are otherwise electrically isolated from each other, whereby the overall capacitance of the array of cells is low, whereby the detection efficiency thereof is substantially equal to a unitary radiation detector, such as detector 12, whose surface area is equivalent to the aggregate area of the cells. However, the signal output from the multicellular detector is far greater than that yielded by the unitary detector. In practice, the parallel-connected detector cells are connected to an output circuit which imposes a reverse bias thereon.

While there have been shown and described preferred embodiments of my invention, it will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention as defined in the annexed claims.

Claims

What I claim is:

1. A radioactive timekeeping standard adapted to produce substantially monoenergetic timing pulses and comprising:

a. a planar array of discrete islands of a radioactive isotope emitting alpha particles, said isotope having a prolonged half-life whose duration is suitable for timekeeping purposes,

b. a solid-state radiation detector having a sensitive surface in parallel relationship to said planar array, the area of said surface being substantially coextensive with the area of said array, and

c. an alpha particle-absorbing mask interposed between said array and said surface and having a matching array of alpha particle-admitting apertures whose geometry is such as to restrict the emanations impinging on the detector surface through the spaces in the apertures to nearly normal angles of incidence and preventing particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island to obviate a spread in energy distribution.

2. A standard as set forth in claim 1, wherein said array of islands is deposited on a backing having protective shielding properties.

3. A standard as set forth in claim 1, wherein said islands are formed of a thin film of radioactive material to prevent a spread of energy distribution.

4. A standard as set forth in claim 1, wherein each aperture in said mask is composed of a first zone adjacent its associated island having a relatively large cross section and a second zone adjacent the detector surface having a constricted cross section.

5. A standard as set forth in claim 4, wherein said second zone of each aperture is flared outwardly in the direction of the detector surface.

6. A standard as set forth in claim 1, wherein said radioisotope is selected from a class consisting of uranium 238, uranium 235, neptunium 237 and plutonium 239.

7. A standard as set forth in claim 2, wherein said backing is a metal disc and said mask is composed of at least one circular plate of the diameter, said detector also having the same configuration.

8. A standard as set forth in claim 7, wherein said mask is composed of two circular plates, one having apertures defining the first zone and the second having apertures defining the second zone.

9. A standard as set forth in claim 1, wherein said detector is formed by an array of individual cells, each disposed to intercept radiation from a respective island, the cells being connected unidirectionally in parallel.

10. A standard as set forth in claim 9, wherein said cells are connected in parallel through diodes and are reverse biased.

11. A radioactive timekeeping standard comprising:

a. a planar array of discrete islands of a radioactive isotope emitting alpha particles, said isotope having a prolonged half-life.

b. a solid-state radiation detector having a sensitive surface in parallel relationship to said planar array, and

c. a mask interposed between said array and said surface, having a matching array of circular apertures whose geometry is such as to restrict the emanations impinging on the detector surface to nearly normal angles of incidence and furthermore preventing particles emanating from any one island from impinging on a neighboring portion of the detector surface associated with another island, wherein the diameter of the circle circumscribing such islands is not greater than twice the distance between the surface of the islands and the plane of the detector and furthermore that the diameter of each of said apertures is not less than the diameter of said circle.