US2816242A - Neutron sources - Google Patents

Description

1957 c. GOODMAN 298162242 NEUTRON SOURCES Filed May 19, 953

IUIUHIIIHIIIHHIIHHIHIHIIIIIIH I3 ZZ HHHIHIIIHHIIHIHIIHHHHHHI I8 E V 27 Dc I 23 5UPPLY INVENTORI CLARK 'GOODM/IN HTTORNE Y5 E l K aterit @tiice 2,8lfi,242 Patented Dec. 10, 1125'? NEUTRON SOURCES Clark Goodman, Boston, Mass., assignor, by mesne ass gnments, to Schlumberger Well Surveying Corporation, Houston, Tex., a corporation of Texas Application May 19, 1953, Serial No. 356,077

11 Claims. (Cl. 31361) The present invention relates to neutron sources and more particularly to novel particle accelerating apparatus for producing neutrons.

The use of charged particle accelerators for producing neutrons has become widespread in the research laboratories and in industry. There is, however, a long felt need for a relatively simple, compact, particle accelerator suitable for portable and relatively rugged field use, as, for example, in neutron well logging operations of the type disclosed in my copending application Serial No. 275,932, filed March 11, 1952, for Neutron Well Logging.

It is an object of the invention, accordingly, to provide a novel particle accelerating apparatus for producing neutrons that is simple, compact and relatively rugged.

Another object of the invention is to provide novel apparatus of the above character for producing neutrons by positive ion bombardment of a target composed of a suitable material.

In accordance with the invention, neutron source means is provided which comprises target means and ion source means for directing high velocity particles to the target means, both disposed in an envelope containing an ionizable gas, the target means being adapted to react with the particles impinging thereon to produce neutrons. The source means comprises electrode means formed with one or more sharp extremities such as needle-like points, or discs or blades having extremely fine or microtonic edges, for example, facing the target means, which electrode means is maintained at a relatively high positive potential with respect to the target means. As is well known, the electrostatic flux density in the vicinity of each of the sharp electrode extremities will be very high. As a result, positive ions will be torn from atoms of the ionizable gas in the envelope which are adsorbed on the sharp extremities of the electrode and will be accelerated by the electric field to a high velocity so that upon impact with the target means, neutrons will be produced.

It will be noted that the hi h velocity particles in the apparatus described above are produced by cold emission and that no heat generating means is involved. Hence, the apparatus is small in size and exceedingly simple in construction so that it is well adapted for use in inaccessible locations such as a bore hole drilled into the earth.

The invention will be better understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

Fig. 1 is a side view, in longitudinal section, of a particle accelerator in a typical associated electrical control circuit, in schematic form, according to the invention;

Fig. 2 is a view, in longitudinal section, of another embodiment of the invention;

Fig. 3 is a side view, in longitudinal section, of a still further embodiment of the invention;

Fig. 4 is a view in transverse section taken along the line 4+4 of Fig. 1, looking in the direction of the arrows;

Fig. 5 is a view in transverse section of the apparatus shown in Fig. 2, taken along the line 55, looking in the direction of the arrows; and

from 20 to kv.

Fig. 6 is a view in transverse section taken along line 66 of Fig. 3 and looking in the direction of the arrows.

Referring now to an exemplary embodiment of the invention shown in Figs. 1 and 4, the apparatus comprises an elongated closed envelope 10 containing an electrode 11 and a target 12. The target 12 consists of a thin coating on the interior of the envelope 10 which is composed, at least in part, of a material which will emit neutrons under positive ion bombardment. For example, the target coating may be a thin layer of zirconium in which a high concentration of tritium has been adsorbed. The envelope 10 also contains a quantity of ionizable gas 16 which may be deuterium gas, for example, at a pressure on the order of from l0 to 10- millimeters of mercury. The envelope 10 must be impervious to gas and is completely sealed so as to maintain the pressure of any gas contained therein at a substantially constant value.

The electrode 11 may include a cylindrical electrically conductive rod or cylinder 13 disposed coaxially with the envelope 10, which also may be composed of some suitable electrically conductive material, such as copper, silver or brass. Connected to and extending from the rod 13 towards the envelope 10 are a number of very fine metallic needles 14, which may be composed of platinum, tungsten, zirconium, iron, copper, silver or other suitable metal. In order to insulate the envelope 10 from the conducting rod 11, a nonconducting disc 15 made of any suitable material, such as Bakelite, may be utilized. The disc 15 also serves the purpose of stabilizing the position of the conducting rod 11 relative to the envelope 1t) and, further, acts to provide a means for sealing the envelope 10.

The electrode 11 is adapted to be maintained at a positive potential in the range 20 to 100 kv. with respect to the envelope 10 and the target 12. To this end, the rod 13 may be connected through the conductors 18 and 22 and a voltage controlling rheostat 21, to the positive terminal of any suitable D. C. high voltage supply 23, the negative terminal of which is connected through the conductors 24, 26 and 19a and a D. C. ammeter 25 to the conductive envelope 10 and to the ground 19. In order to obtain indications of the potential ditference between the target 12 and the electrode 11, a conventional D. C. voltmeter 27 may be connected between the conductors l8 and 26.

As an example of the simplicity and compactness of the neutron generating apparatus of the invention, the following order of dimensions may be utilized in practice for the typical embodiment shown in Fig. l: the cylindrical rod 13 may be on the order of 2 to 5 millimeters in diameter and 5 to 10 centimeters in length; the metallic needles 14 may be about 1 centimeter in length with a tip radius on the order of 10*" to 10* centimeters; and the needles 14 may be spaced evenly about the rod 13, there being approximately 100 needles per centimeter of rod length.

In operation, the rheostat 21 is adjusted until the potential difference between the electrode 11 and the target 12 has a desired value in the range from 20 to 100 kv., as indicated by the meter 27. Under these conditions, an extremely high intensity electrostatic field obtains in the immediate vicinity of the tips of the needles 14. As molecules or atoms of deuterium gas alight on the tips of the needles 14, the high intensity field prevailing there causes positive ions (deuterons) to be torn off which flow to the conductive envelope 10 which is at a negative potential. In passing to the envelope 10, the deuterons are accelerated by the electric field so that when they reach the target they will have acquired energies corresponding to the potential difierence between the electrode 11 and the envelope 10 which, as stated, lies in the range Upon impact with the target 12,

the deuterons react with the tritium to produce neutrons of 14 m. e. v. energy level.

An alternate embodiment of the invention is shown in Fig. 2, which utilizes a plurality of mic'rotomic edged blades 28, in contrast to a plurality of needles 14 as in the embodiment shown in Fig. 1. The blades 28 may be positioned so as to lie in angularly spaced radial planes through the longitudinal axis of the rod 13, as shown. The blades 28 may be secured to the rod 13 in any suitable manner as byspot welding, brazing or silver soldering, for example. Alternatively, the blades 28 could be inserted in longitudinal slots formed in the outer surface of the rod or cylinder 13 by a process of shrink fitting.

Still another embodiment of the invention may utilize, as shown in Figs. 3 and 6, a plurality of longitudinally spaced apart transverse circular discs 29 having microtomic edges. The discs 29 may be attached to the conducting rod or cylinder 13 in any suitable manner, such as described with relation to the blades 28 in Fig. 2.

Thus, it is seen that a compact, relatively rugged and easily controllable neutron source is provided in accordance with the invention. By using an electrode having elements provided with sharp extremities maintained at a relatively high positive potential, particles for bombarding a target may be produced and accelerated to desired energy levels with a minimum of equipment, so that the apparatus can be made small in size and rugged in construction.

It will be understood that, for prolonged operation of the neutron generating apparatus of the invention, appropriate pumping or getter devices will be required to maintain the gas 16 in the proper condition for effective operation.

It will be further understood that reactions other than the D-T reaction may be utilized to produce neutrons in the apparatus described above. For example, by using a target 12 comprising deuterium instead of tritium, neutrons could be produced by the D-D reaction. Other reactions might be used such as, for example, one of the following:

(1) The T-D reaction:' This is they inverse of the D-T reaction. Because of the heavier mass of T, the voltage required for a given yield of neutrons is times that for D-T. The spectrum of neutrons is the same, essentially monoenergetic at 14 m. e. v.

(2) The T-T reactionz This reaction goes two Ways, viz (a) T(t,ot)2n, (1:11.32 rn. e. v.; and (b) T(t,n)He (1:10.32 rn. e. v. Reaction a leads to a continuum of a particles and neutrons. Reaction [1 yields approximately monochromatic groups of neutrons, corresponding to the ground and excited states of He plus a continuum of neutrons and a particles from the subsequent breakup of He'. 7

(3) The Li (d,n)He -l-I-Ie reaction (Q=l.79 rn. e. v.):

A broad resonance for neutron production at 90 is observed at 0.41 m. e. v.; a further sharp resonance appears at 2.12 rn. e. v.

(4) The Li (t,n)He +Hc reaction (Q=l6.10 m. e. v.): This reaction has a low crosssection and yields a continuum of neutrons and a particles.

(5) The Li"(p,n)Be' reaction (Q: l.645 m. e. v.): This reaction has a large yield of low energy neutrons above threshold at 1.882 m. e. v., the maximum yield being at 2.22 m. e. v.

(6) The Li' (d,n)l3e reaction (Q=l5.0l7 rn. e. v.; This reaction yields neutrons of many different energies because of the several excited states of Be formed.

(7) The T(p,n)He reaction (Q: 0.764 m. e. v.): This reaction gives a copious yield of low energy monoenergetic neutrons above the threshold of 1.019 m. e. v. M (8) The H (t,n')He reaction (Q=0.764 m. e. v.): This reaction is the inverse of (7) but requires .3 times the voltage for the same yield because of the 3 times larger mass of the tritium ions.

4. Other reactions which might be mentioned but are of less practical interest below 2 m. e. v. are:

Be (p,n)B Q: 1.85 m. e. v. Li"(t,2nd)Be Q=8.759 m. e. v. Li (t,n)Be Q=l0.425 rn. e. v. Li (t,n)He +He Q=7.86 rn. e. v. Be (d,n)B Q=4.360 m. e. v. Be (I-Ie ,n)C Q=7.563 m. e. v. B (a',rt)C Q=6.472 m. e. v. Be (o,n)C Q=5.708 m. e. v. B (d,n)C Q=13.724 m. e. v.

The above-described embodiments are intended to be merely exemplary and they are obviously susceptible of modification and variation Within the scope and spirit of the invention as defined in the appended claims. For example, the envelope 10 could be constructed of glass having a metallic electrically conductive coating thereon. The needles or microtome edges could be coated with zirconium deuteride to increase the production of deuterium ions. The target could be LiT or other chemical combinations of two or more neutron-producing elements. Further, in lieu of the zirconium coating, other metals such as uranium or titanium may be used.

I claim:

1. A neutron source comprising an envelope containing an ionizable gas, an electrode within said envelope formed with one or more sharp extremities, an electrically conductive target within said envelope and spaced from said electrode, said electrically conductive target being composed, at least in part, of material which will emit neutrons under positive ion bombardment, and means for maintaining said target at a negative potential with reference to said electrode, whereby positive ions of said gas are produced by the electrical field adjacent said one or more sharp extremities and are propelled against said target to produce neutrons.

2. A neutron source comprising an envelope containing an ionizable gas at a pressure in the range of 10- to 10 millimeters of mercury, an electrode within said envelope formed with one or more sharp extremities thereon, an electrically conductive target Within said envelope and spaced from said electrode, said electrically conductive target being composed, at least in part, of material which will emit neutrons under positive ion bombardment, and a source of potential of at least 20,000 volts connected between said target and said electrode to maintain a high intensityelectrostatic field adjacent said one or more sharp extremities.

3. A neutron source comprising an envelope containing deuterium, an electrode within said envelope formed with one or more sharp extremities thereon, an electrically conductive target within said envelope and spaced from said electrode, said electrically conductive target be ing composed, at least in part, of material which will emit neutrons under bombardment by deuterons, and a source of potential connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more sharp extremities.

4. A neutron source comprising an envelope containing an ionizable gas, an electrode within said envelope formed with one or more needle-like points thereon, an electrically conductive target within said envelope and spaced from said electrode, said electrically conductive target being composed, at least in part, of material which will emit neutrons under positive ion bombardment, and a source of potential connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more needle-like points.

5. A neutron source comprising an envelope containing an ionizable gas, elongated electrode means within said envelope comprising one or more blades having microtomic edges and extending longitudinally of said envelope, an electrically conductive target within said envelope and spaced from said electrode means, said electrically conductive target being composed, at least in part, of material which will emit neutrons under positive ion bombardment, and a source of potential connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one .or more microtomic edges.

6. A neutron source comprising an envelope containing an ionizable gas, elongated electrode means Within said envelope comprising one or more transversely extending disc means having microtomic edges, an electrically conductive target Within said envelope and spaced from said electrode means, said electrically conductive target being composed, at least in part, of material which will emit neutrons under positive ion bombardment, and a source of potential connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more microtomic edges.

7. A neutron source comprising an envelope containing deuterium gas at a pressure in the range from l0 to millimeters of mercury, an electrode within said envelope formed with one or more sharp extremities, an electrically conductive target within said envelope and spaced apart from said electrode, said electrically conductive target being composed, at least in part, of tritium, and a source of potential of at least 20,000 volts connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more sharp extremities, whereby deuterons, produced adjacent said sharp extremities and accelerated by said electrostatic field, will bombard and react with said target to produce substantially monoenergetic neutrons.

8. A neutron source comprising an envelope containing deuterium gas at a pressure in the range from 10- to 10* millimeters of mercury, an electrode within said envelope formed with one or more sharp extremities, an electrically conductive target within said envelope and spaced apart from said electrode being composed, at least in part, of tritium absorbed in Zirconium, and a source of potential of at least 20,000 volts connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more sharp extremities, whereby deuterons, produced adjacent said sharp extremities and accelerated by said electrostatic field, will bombard and react with said tritium to produce neutrons.

9. A neutron source comprising an envelope containing deuterium gas at a pressure in the range from 10- to 10 millimeters of mercury, an electrode within said envelope formed with one or more needle-like points thereon, an electrically conductive target within said envelope and spaced apart from said electrode being composed, at least in part, of zirconium treated with tritium, and a source of potential of at least 20,000 volts connected between said target and said electrode to maintain a high intensity electrostatic field adjacent said one or more needle-like points, whereby deuterons, produced adjacent said needle-like points and accelerated by said electrostatic field, will bombard and react with said tritium to produce neutrons.

10. A neutron source comprising an envelope containing deuterium gas at a pressure in the range from 10 to 10- millimeters of mercury, electrode means within said envelope comprising one or more blades having micromatic edges and extending longitudinally of said envelope, an electrically conductive target within said envelope and spaced apart from said electrode means, being composed, at least in part, of tritium, and a source of potential of at least 20,000 volts connected between said target and said electrode means to maintain a high intensity electrostatic field adjacent said one or more microtomic edges, whereby deuterons, produced adjacent said microtomic edges and accelerated by said electrostatic field, will bombard and react with said tritium to produce nuetrons.

11. A neutron source comprising an envelope containing deuterium gas at a pressure in the range from 10* to 10- millimeters of mercury, electrode means within said envelope comprising one or more transversely extending disc means having microtomic edges, an electrically conductive target within said envelope and spaced from said electrode means, said electrically conductive target being composed, at least in part, of tritium, and a source of potential on the order of 20,000 volts connected between said target and said electrode means to maintain a high intensity electrostatic field adjacent said microtomic edges, whereby deuterons, produced adjacent said one or more microtomic edges and accelerated by said electrostatic field, will bombard and react With said target to produce substantially monoenergetic neutrons.

References Cited in the file of this patent UNITED STATES PATENTS 2,231,877 Bennett Feb. 18, 1941 2,251,190 Kallmann July 29, 1941 2,645,742 Reeves July 14, 1953 FOREIGN PATENTS 491,766 Great Britain Sept. 8, 1938

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