Superconducting cyclotron neutron source for therapy

Abstract

A neutron source for medical therapy purposes comprising a cyclotron comprising an iron metal housing acting as a magnetic yoke, magnetic shield, radiation shield, and vacuum vessel, a pair of superconducting coils mounted in a cavity in the housing said coils being cooled to superconducting temperatures by passage of a refrigerant fluid therethrough and connected to an electrical energy source such that a high current flows in the coils producing an intense magnetic field inwardly between the coils, an ion orbiting region defined by pairs of sectoral-shaped RF electrode structures energized at an RF frequency and focussing flutter poles mounted in the intense magnetic field between coils, a source of ions positioned centrally of the ion orbiting region to provide a stream of ions that will be orbited in the orbit region; an ion target positioned internally of said iron housing at an outer position in the orbit region such that orbiting ions strike the target and produce neutrons; a channel formed in the iron housing from the target to the exterior for passage of the beam of neutrons formed at the target, said channel acting as a beam collimator; and a mounting structure for movably mounting the cyclotron and target such that the neutron beam produced can be employed at more than one position.

Description

This invention relates to a superconducting cyclotron neutron source for medical therapy purposes.

Recent experiments in the irradiation of tumors by neutrons have given encouraging results. One series of experiments were done with a 16 MeV deuteron beam on a beryllium target approximately 100 mg/cm.sup.2 thick. This provided a neutron spectrum with a mean energy about 8 MeV and a "depth-dose" of 43% at 10 cm. The does rate at a distance of 120 cm from the target for a 100 .mu.A beam was about 50 rad per minute requiring 2.4 minutes for the usual required fraction of 120 rads. Although these results are quite good it is considered that higher mean neutron energy would improve the treatments. A depth dose 50% at 10 cm would appear to be desirable. This can be achieved with 14 MeV neutrons from a d-T (deuterium beam-tritium target) source at a source distance of 75 cm. A d-T source producing 5 x 10.sup.12 n/sec. gives a dose rate of 30 rad/min. at 75 cm. The difficulty with d-T sources is in the target technology required to obtain higher dose rates, say 50 rad/min., with a reasonable target life. The present target lifetimes are unacceptable.

A cyclotron source can provide the desired dose rate, say 120 rad/minute for one minute irradiations to obtain the desired depth-dose from a thin beryllium target requires a deuteron beam with an energy of about 25 MeV. Conventional designs of cyclotrons capable of producing 25 MeV deuterons are very heavy and bulky (about 115 tonnes in weight). It is highly desirable that a neutron source for this purpose have an isocentric mounting for multiple beam directions in fractionated treatment. This would suggest the mounting of the neutron source on a gantry similar to that used for cobalt irradiators. it appears from this that a conventional cyclotron producing the required beam energy would have to have its beam extracted and transported to an isocentrically mounted target and this is out of the question for this application and would be uncompetitive with the d-T type source which is relatively small and which can be readily mounted in an isocentric manner.

It is therefore an object of the present invention to provide a neutron source for medical therapy that has high intensity is relatively light in weight, and which can be readily mounted isocentrically.

This and other objects of the invention are achieved by a neutron source comprising a cyclotron using an air core superconducting magnet to provide high intensity magnetic fields for producing a relatively high energy beam of ions, a target made of a material chosen from the group lighium, beryllium positioned in the ion beam path such that a high energy neutron beam is produced, and an isocentric mounting for said cyclotron and target.

A superconducting cyclotron of a type suitable for the present purpose is described in applicants' pending application Ser. No. 419,034.

In drawings which illustrate an embodiment of the invention,

FIG. 1 is a cross section of a superconducting cyclotron on an isocentric mounting,

FIG. 2 is a cross section of the cyclotron showing the neutron beam from the internal target,

FIG. 3 is a graph of the mid phase magnetic field of the cyclotron with and without flutter poles,

FIG. 4 is a cross section of the cyclotron and the target,

FIG. 5A and 5B are midplane and vertical cross sections of the RF accelerating structure, and

FIG. 6A and 6B are vertical and midplane cross-sections of the ion source region.

A first type of neutron reaction considered for this apparatus is the .sup.9 Be (d,n).sup.10 B with a Q of 4.35 MeV. At these energy levels, the stripping reaction predominates so the output neutrons have about half of the deuteron energy. For a thick target the mean energy is about 0.36 E.sub.d. It has been found that a mean energy of about 14 MeV can be obtained with 25 MeV deuterons if the beryllium target is made about 50 mg/cm.sup.2 in thickness. The neutron yield is then approximately one-third that for a thick target requiring beam intensities about three times larger. Other types of neutron reaction is .sup.7 Li(d,n).sup.8 Be with Q of 15.0 MeV and .sup.9 Be(P,n).sup.9 B with Q = 1.9 MeV. The neutron yield from this reaction is a little smaller (80% of the beryllium target yield at 16 MeV) but the neutron spectrum has a higher mean energy and a component going to much higher energies. From known data, it is considered that a 25 MeV, a thick lithium target would have about twice the yield of a 50 mg/cm.sup.2 beryllium target, a mean energy of approximately 16 MeV and about 1/3 of the neutrons in a distribution extending to 26 MeV. These high energy neutrons would be very effective in the C.sup.12 (n,n.sup.1) 3.sup.4 He and O.sup.16 (n,n.sup.1) 4.sup.4 He reactions which are believed to be important in therapy.

Referring to FIG. 1 a cyclotron 10 is mounted isocentrically by supports 11a and 11b on a strong, vertical, rotating pillar 11 such that the neutron beam 12 of the device can be moved over a fairly long arc. The cyclotron has two superconducting coils 13 and 14 mounted in an iron housing 15 that has the combined function of a magnetic yoke, magnetic shield, radiation shield, and vacuum vessel. The coils are mounted in a cavity in the iron housing in a radiation shield 16. Adjustments to the positioning of the coils can be made by means of a supporting and adjusting elements 17a, 17b, 17c, and 17d. Helium cooling fluid and electrical leads to the coils pass through port 18 into the interior of shield 16. The coils are Nb.sub.3 Sn superconductors providing a 7 tesla average magnetic field in orbit region 19 having a BR product of 1 for a deuteron output energy of 25 MeV (B = field strength and R = radius). An ion source 20 provides a stream of ions (deuterons or protons) that are introduced into the orbit region via passage 21. These ions orbit outwardly in the orbital region between plates 22 and 23 forming an RF resonator 36 mounted on RF input structures 24 and 25 and energized from an appropriate RF source via line 27. A vacuum pump 28 connects to the internal cyclotron cavity via line 29 to provide the necessary vacuum level required for cyclotron operation. A target 30, either beam path inside the cyclotron as shown through port 31. Neutrons produced at the internal target pass through window 33 are collimated by a channel 32 in the magnet support spool and the iron yoke 15.

Fig. 2 shows the described features and more detail of the four sector flutter pole geometry (sectors 34a, 34b, 34c, 34d) and the RF accelerating structure (resonator) 36. The flutter poles provide azimuthal focussing and although four sectors are illustrated, other numbers e.g., three might be used. These are formed as raised portions of the iron housing structure leaving in effect four valley sectors 35a, 35b, 35c, 35d). The superconducting coils in conjunction with the flutter poles provide an isochronous field profile to an outer radius R.sub. 0. Since the energy level is fixed by design superconducting trim coils or normal trim coils are not required. RF accelerating electrodes 36 are positioned in valleys 35a and 35c and are mounted on "leg" 24 and 25 extending axially to form a .mu./2 resonator at 212.8 MHz, four times the cyclotron frequency of 53.2 MHz. A large energy gain per turn is not required since the beam is not extracted and orbit separation is not necessary. The ions from the central ion source are extracted by the RF field and this determines the RF voltage required. Typical beam power for a 200 .mu.A beam is 5 KW. An RF power of 10 KW will drive the electrodes to about 20 KV peak with full beam.

The total weight of the cyclotron of beam power levels described above is about 4 tonnes. This compares favorably with the weight of shielding required for an isocentrically mounted cobalt-60 therapy head. The design provides a minimum of 30 cm. of iron shielding.

FIG. 3 shows the required radial field profile for 25 MeV deuterons at a final orbit radious (R.sub.o) of 15 cm. The midplane magnetic field for isochronism is curve A. Because the flutter poles enhance the midplane field and also cause some radial variation in the azimuthally averaged field, the field profile required from the magnet coils is as shown by curve B which is the isochronous field with flutter pole field substracted.

FIG. 4 is a cross-section through the cyclotron showing the RF resonator 24 on one side and a flutter pole 34a on the other. The latter is formed as part of the iron shield and vacuum vessel structure 15.

FIG. 5A and 5B shown the reasonator cavity 36 more clearly. Two interconnected sectors are mounted in opposite valleys and joined at the centre to form a single .mu./2 resonator. Two sectors instead of four give a simpler structure at the ion source. The resonator is energized at RF by a probe loop 39 attached to leg 24 which may be a tube or rod. Other methods of energizaton may be used. A large energy gain is not necessary but at least two sectors are necessary to provide symmetry. An rf power of .about. 10 KW should be sufficient to run the resonator at about 25 KV peak and supply a beam power up to about 5 KW (200 .mu.A at 25 MeV). A fixed frequency system comprisng a stable oscillator, intermediate amplifier and power amplifier with 0.2 stability in power level and frequency would be adequate.

FIG. 6A and 6B show vertical and midplane sections of the ion source 21. A tungsten filament arc source 37 in conjunction with a D.sub.2 gas stream provides a supply of deuterons into the orbit region 19. If protons are used as the ions, then an H+ gas stream would be used. A repeller electrode 38 causes the ion beam to travel in the correct paths to begin orbiting.

The azimuthal focussing is provided by a flutter field produced by iron poles extending to the end sheild which then forms a yoke. (See FIGS. 2 and 4). The estimated field increase between the poles .DELTA.B = 1.6 T giving for an average field, B, a flutter factor ##EQU1## for h .apprxeq. 0.5 . At the extraction orbit .gamma.= 1.0133 for 25 MeV deuterons. Using ##EQU2## gives a maximum spiral angle .xi. = 45.degree.. The flutter field cannot extend inward to the ion source so a magnetic "hill" is used to produce "weak" focussing out to where the flutter focussing becomes effective. This can be provided by a suitable rion "shim" in the central region. The radial betatron frequencey .nu..sub.r = .gamma. in the isochronous region reaching a maximum value of 1.0133. In the central hill region however .nu..sub.r <1 and must pass through unity at a small radius where imperfections could drive the .nu.= 1 resonance. This is not expected to cause difficulty since it would only spread the beam radially at the target.

The neutrons are generated in a 50 mg/cm.sup.2 thick beryllium metal layer on a 1 mm gold backing. Stopping the deuterons in gold after they pass through the beryllium reduces the low energy neutron flux. The gold is in turn mounted on a water cooled copper backing. The power in the beam is as large as 5KW giving a power density of .about. 1 KW/cm.sup.2 if the beam can be spread over 5 cm.sup.2. This is manageable but will require a carefully designed coolant channel with a water flow of about 2 gal. per minute. Under normal treatment conditions at 120 rad per one minute fraction, the fractions will be repeated at 10- 15 minute intervals i.e., the duty cycle is .about. 10%. The target assembly is mounted on a tube extending from the outside of the shielding and arranged so that it can be withdrawn into a vacuum lock for servicing.

In a typical design, a pair of coils with an inside diameter of 36.5 cm, a cross section of 12 cm x 12 cm and a separation of 13 cm, provides a field with approximately the correct magnitude and shape. There are 1.8 x 10.sup.6 ampere turns in each coil. This corresponds to an overall current density of 12,500 A/cm.sup.2, which is within the state of the art for niobium-tin coils. The maximum radial field is 3 Tesla. The increase near the central region in the required coil field shown in curve B of FIG. 3 results from the flutter poles not extending to the cyclotron centre.

Lorentz forces within the coils are large but tractable. The maximum hoop stress, which occurs at the inside winding is approximately 30,000 psi assuming no transfer of the radial force to the outer windings. If such transfer is assumed so that a uniform hoop stress is realized, then its value would be no greater than 15,000 psi. The tensile strength of stainless steel is considerably larger than this at low temperatures so no problems are expexted here. Two types of superconductor may be used in winding the coils, Nb.sub.3 SN and V.sub.3 Ga. The first of these is commercially available and has a guaranteed short sample critical current of 600 amperes at 10 Tesla and 4.2K. The conductor has the Nb.sub.3 Sn vapor deposited on a Hastelloy substrate and is clad on both sides with copper. Its dimensions are 0.5 inch wide .times.0.0072 in. thick. With this conductor, a coil as illustrated in FIG. 4 is made up of 9 pancake windings each with 400 turns. The total length of superconductor to wind two coils is then 11,300 metres. Total weight of superconductor is 210 Kgm and the weight of both coils with an inter coil bridge is 400 Kgm.

The coils are cooled by circulating supercritical liquid helium at 4.2 K around them. The advantages of supercritical helium at a pressure above the critical value of 2.26 atm.,) as a cooling medium have been known but have not yet been extensively exploited. For the present application the advantages are twofold; first high heat transfer coefficients are possible and second no difficulties caused by two phase flow of the helium can occur, since it is always maintained in the liquid state. The disadvantage includes the requirement that the helium vessel around the coils must support a pressure greater than 3 atmospheres - 4 atmospheres seems to be a good design value.

Claims

We claim:

1. A neutron source for medical therapy purposes comprising:

a. a cyclotron comprising an iron metal housing acting as a magnetic yoke, magnetic shield, radiation shield, and vacuum vessel, a pair of superconducting coils mounted in a cavity in the housing said coils being cooled to superconducting temperatures by passage of a refrigerant fluid therethrough and connected to an electrical energy source such that a high current flows in the coils producing an intense magnetic field inwardly between the coils, an ion orbiting region defined by pairs of sectoral-shaped RF electrode structures energized at an Rf frequency and focussing flutter poles mounted in the intense magnetic field between coils, a source of ions positioned centrally of the ion orbiting region to provide a stream of ions that will be orbited in the orbit region;

b. an ion target positioned internally of said iron housing at an outer position in the orbit region such that orbiting ions strike the target and produce neutrons;

c. a channel formed in the iron housing from the target to the exterior for passage of the beam of neutrons formed at the target, said channel acting as a beam collimator; and

d. a mounting structure for movably mounting the cyclotron and target such that the neutron beam produced can be employed at more than one position.

2. A neutron source as in claim 1 wherein the ions orbited in the cyclotron are deuterons and the target is a thick beryllium metal layer.

3. A neutron source as in claim 1 wherein the ions orbited in the cyclotron are deuterons and the target is a thick lithium metal layer.

4. A neutron source as in claim 1 wherein the mounting structure is a gantry allowing movement of the cyclotron and thus the neutron beam in an isocentric path.

5. A neutron source as in claim 1 wherein the flutter poles are four in number and are formed as spiral-edged sectoral raised portions of the iron housing structure defining valleys between with the said RF electrode structures being positioned in the valley regions.

6. A neutron source as in claim 1 wherein the refrigerant fluid is supercritical helium at a temperature of 4.2K.