Performance of a high-pressure, high-beam-current gas target

NUCLEAR

INSTRUMENTS

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METHODS

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NORTH-HOLLAND

PUBLISHING

CO.

PERFORMANCE OF A HIGH-PRESSURE, HIGH-BEAM-CURRENT GAS TARGET* JOHN DAVID CARLSON

Physics Department, Ohio University, Athens, Ohio 4570l, U.S.A. Received 23 M a y 1973 A low-mass, small-volume gas target which utilizes O-rings for m o u n t i n g the entrance window is described. T h e target is intended as a n e u t r o n source using the D(d,n)~He, T(d,n)4He

a n d T ( p , n ) J H e reactions. T h e p e r f o r m a n c e o f the gas target with respect to gas pressures a n d b e a m intensities is discussed.

1,, Introduction A program has recently been underway at the Ohio University Accelerator Laboratory to develop a highpressure gas target for use as a neutron source, ul:ilizing the D(d, n)3He, T(p, n)3He, and T(d, n)4He reactions. Such a gas target is needed to effectively carry out studies of elastic and inelastic neutron sc,attering, neutron-induced charged particle (n, z) reactions, and direct neutron-capture reactions in the energy range up to 28 MeV. The Ohio University 11-MeV Tandem Accelerator is capable of producing momentum-analyzed beams of protons or deuterons with intensities in excess of 80/~A. The desire to be able to use as large a portion of the available beam intensity as possible without causing an excessive neutron-energy spread and neutron background placed stringent requirements on the gas target and, in particular, the entrance window of such a target. Many gas-cell designs have been reported ~ 6). Typically, these gas cells consist of a thin walled tube

with a flange at one end to which the entrance window is attached. The other end of the tube is closed and serves as the beam stop or else is an exit similar to the entrance window. Generally, gas cells may be grouped into one of two categories, depending on the method of entrance-window attachment. The first category includes those designs in which an entrance foil is rigidly attached to the cell body by either solder or a cement such as epoxy. In the second category fall those cells in which O-rings are employed to accomplish entrance-foil attachment. The performance of a particularly simple gas cell which used either solder or epoxy has been investigated by Morris and Thornton~). They investigated the beam-intensity limitations of molybdenum and Havar foils using a 3.5 MeV alpha-particle beam. From their alpha-particle results they extrapolated the Mowindow performance to proton and deuteron beams as a function of bombarding energy. They found soldered Mo foils to be the most favorable.

* W o r k s u p p o r t e d in part by the U.S. A t o m i c Energy C o m mission.

2. Experimental In the present work, the performance of a gas target

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Fig. 1. Cross sectional view o f the gas target, aperture, m o u n t i n g a n d electron-suppression ring. W h e n fully assembled, the knurl nuts compress the O-rings until metal-to-metal contact o f all parts along the assembly is obtained.

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J . D . CARLSON

which uses a n O-ring foil a t t a c h m e n t method was investigated. The design of the gas cell used in the present experiment is similar to a gas cell which has been used at Los A l a m o s Scientific Laboratory2). The gas cell, entrance collimator, electron-suppression ring and m o u n t i n g are illustrated in fig. 1. The cell is 3.1 cm long and 0.96 cm in diameter. The cell body is fabricated from 0.25 m m thick stainless steel tubing. A 0.25 m m thick p l a t i n u m disc is silver soldered into one end of the tube to serve as a beam stop. A threaded stainless-steel flange having a 0.5 cm diameter aperture is silver soldered to the other end of the tube. A 60 cm length of 1.0 m m O.D. by 0.5 m m I.D. stainless-steel hypodermic tubing is silver soldered into the cell wall to provide the connection with the gas h a n d l i n g system. The entrance foil is sandwiched between a pair of Viton O-rings*. W h e n the knurl nut which holds the assembly together is tightened, the O-rings are compressed until metal-to-metal contact of the foil with the supporting structure is obtained. This allows heat to be carried away from the foil and, also, provides a very symmetric and uniform support. It has been the a u t h o r ' s experience that while a solder method of foil a t t a c h m e n t may provide conductive cooling superior to the O-ring method, uneven solder joints can have a tendency to tear the foil. Furthermore, the O-ring support allows reverse flexing of the foil and removes the uncertainty of solder or flux protruding into the window aperture. One detail of the O-ring supporting structure should be emphasized. The inner edges of the stainless-steel pieces which contact the foil as it flexes or reverse * Obtained from Edwards High Vacuum Inc., Grand Island, New York, U.S.A.

flexes are slightly rounded (radius ~ 0 . 2 ram) in order that the foil does not bear against a sharp edge and tear due to the high stress concentration. The entire target assembly shown in fig. 1 attaches to the end of the beam line. The Ta aperture has a diameter of 0.4 cm and prevents beam from striking the electron suppressor a n d O-ring support structure. The collimator also serves to insure that the beam is not too finely focused by requiring that the beam spot be diffuse enough that a small current is read from the collimator (typically ~ 1 % of total beam)*. The electron suppressor is m a i n t a i n e d at a bias of - 5 0 0 V with respect to ground to prevent back-streaming of electrons from the target. The target is cooled by spraying the end of the cell with an air-water mist. The procedure for determining the beam-intensity limitations of the entrance foils was as follows. The cell was filled with 1100 tort of D 2 gas. A 4 MeV, dc p r o t o n beam with an initial intensity of 5 p A was passed through the cell. The beam intensity was then increased in approximately 2 . 0 p A steps every 20 minutes until the foil failed. F o r each type of foil several repeat tests were made to verify the reproducibility of the results. Foils made of Mo +, W +, Havar**, a n d Ni** were 1 The beam is initially focused with a quartz target mounted in place of the gas-target assembly. The quartz target has a 0.4 cm dia. aperture in its center. The beam is steered and focused such that ~99% passes through the hole in the quartz and the remainder evenly illuminates the periphery of the hole. + Obtained from Goodfellow Metals, Ltd., Ruxley Towers, Claygate-Esher, Surrey, England. **Obtained from Hamilton Metal Science, Lancaster, Pa., U.S.A. **Obtained from Chromium Corp. of America, Cleveland, Ohio, U.S.A.

TABLE I

Beam currents through various gas cell foils for a 4 MeV dc proton beam passing through a gas cell filled with 1100 torr of D2 gas. Foil type

Mo Mo W Havar Ni

Thickness (/~m)

(keV)a

Maximum current without permanent pin-hole leak QtA)

5.0 10.0 8.0 2.5 2.5

222 444 513 114 120

20 l0 17 10 <5

Power dissipated in foil at maximum current (W)

Current at onset of diffusion leakb Q~A)

4.4 4.4 8.7 1. I <.~0.6

15.5 __c 15 6.5 t.

a Thickness as expressed in terms of energy loss for a 4 MeV proton transversing the foil. b Pressure in gas cell falling at rate of ~40 torr/hr which was equivalent to a gas loss rate of ~ 5 x 10 5 cm3/sec. e A permanent leak developed before any diffusion was observed.

HIGH-PRESSURE,

HIGH-BEAM-CURRENT

tested in the above manner. The results are given in table 1. Except for the Ni cases, none of the tbils failed by rupture. In all cases a slow permanent leak developed which upon subsequent visual inspection proved to be a small pin-hole melted through the foil. The results Of repeated tests were found to be repeatable within + 10%. As indicated in table 1, the tests also showed that at beam intensities somewhat less than lhe maximum without a permanent leak a leak of another sort developed. This second type of leak has been attributed to gas diffusing through the hot foil. These leaks were non-permanent in that they stopped if the beam was turned off or reduced. Table 1 lists the beam intensities at which these diffusion leaks lirst became apparent. In these instances the pressure in the cell started dropping at a rate of approximately 40 torr/hr which was equivalent to a gas-loss rate of approximately 5 x 10 -5 c m 3 / s . At this leakage rate a rise in pressure in the beam-line vacuum became noticeable. It should be noted that the maximum beam intensities were sensitive to how sharply the beam was focused. For the results reported in table 1 the beam spot was diffused sufficiently to fill the collimator aperture. It was possible, however, by adjusting the last quadruple-doublet focusing magnet of the beamhandling system to bring the beam to a much sharper focus ( ~ 0.1 cm in diameter). In this case the foils would develop pin-hole leaks at approximately 1/2 the beam intensities given in table 1. The Mo and Havar foils were also tested to determine the maximum gas pressure at which they could be operated with beam passing through them. These tests were made by initially filling the cells with 1100 torr of D 2 gas. A 4 MeV dc proton beam with an intensity in each case of approximately 1/2 the maximum listed in table 1 was passed through the cell. Every 15 minutes the gas pressure was increased by approximately 500torr until the entrance foil ruptured. The results of these tests are shown in table 2.

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GAS TARGET TABLE 2

G a s pressure limits with a 4 MeV dc p r o t o n b e a m passing t h r o u g h a gas cell filled with D2 gas. Foil type

Mo Mo Havar

Thickness (/~m)

Beam intensity

(ItA)

Maximum pressure (torr)

10.0 5.0 2.5

5.0 10.5 7.0

4800 a 3100 2600

T h e limit o f the regulator on the D2 bottle was 4800 torr. T h e b e a m intensity was subsequently raised to 11.5/zA without a foil rupture.

found in the pressure tests summarized in table 2. The pressure capability tests on the Mo foils showed that increased thickness allowed operation of higher pressures. Thus, if high pressure operation is necessary, the results indicate that a compromise between increased thickness for more pressure and decreased thickness for more beam intensity is necessary. From consideration of the maximum power dissipated in each of the foils in table 1 it is possible to learn still more. The table shows that the W foils were superior to all others in power dissipation. In fact, they surpassed by almost a factor of 2 the Mo foils. Based on the relative performance of the two thicknesses of Mo foil it is possible to extrapolate the performance of W foils to thicknesses less than the 8 pm used in the present work. For a 4 pm W foil one arrives at a maximum beam intensity in excess of 30 pA for 4 MeV protons. The only apparent difficulty with such foils is one of availability. The author has been unable to locate a manufacturer willing and able to make pin-hole free W foils less than 8 pm thick. The author wishes to acknowledge the assistance of N. K. Lambha in the testing of these gas cells.

3. Discussion

References

From table 1 it is seen that of the foils tested the 5 pm Mo foil is able to handle the largest amount of beam without a leak. Furthermore, it is seen from a comparison of the two thicknesses of Mo foil that the maximum beam intensity is inversely proportional to the foil thickness. In fact, the maximum power dissipated in the Mo foils is found to be the same for both thicknesses, 4.4 W. Thus, if one considers only Mo foils and ignores cell-pressure considerations, one is led to the conclusion that the thinner the foil the better. This is, however, just the opposite of the result

1) C. L. Morris a n d S. T. T h o r n t o n , Nucl. Instr. a n d Meth. 96 (1971) 281. 2) D. K. McDaniels, I. Bergqvist, D. Drake a n d J. T. Martin, Nucl. Instr. a n d Meth. 99 (1972) 77; a n d J. T. Martin, private c o m m u n i c a t i o n . 3) D. S. C r a m e r a n d L. Cranberg, Nucl. Instr. a n d Meth. 93 (1971) 405. 4) j. S. C. McKee, Nucl. Instr. a n d Meth. 57 (1967) 179. 5) B. H'olmqvist a n d T. Wiedling, Nucleonik 6 (1964) 183. 6) j. H. Coon, Fast neutron physics, part I (eds. J. B. Marion a n d J. L. Fowler; lnterscience, New York, 1960) p. 677, a n d references therein.