Development and use of a thinfilm transmission ...

Development and use of a thinfilm transmission positron moderator K. G. Lynn, B. Nielsen, and J. H. Quateman Citation: Appl. Phys. Lett. 47, 239 (1985); doi: 10.1063/1.96231 View online: http://dx.doi.org/10.1063/1.96231 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v47/i3 Published by the AIP Publishing LLC.

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Deve~opment

and use of a thin-film transmission positron moderator

K. G. Lynn and B. Nielsen Physics Department. Brookhaven National Laboratory. Upton. New York 11973

J. H. Quateman Francis Bitter National Magnet Laboratories. Massachusetts Institute of Technology. Cambridge. Massachusetts 02139

(Received 15 April 1985; accepted for publication 14 May 1985) A narrow energy beam of slow positrons can be generated by moderating the beta-decay spectrum of a radioactive /3 + source. We report here the development and use of the first moderator in which the low-energy positrons are not extracted from the surface facing the/3 + emitter but from the opposite surface, i.e., the transmission moderator. An advantage is that such a configuration eliminates the problem of moderated positrons passing by the radioactive source. This is an essential problem when the source is physically large such as the present commerically available Na 22 sources. This moderator is a 5000-A self-supporting W (1 (0) film. The growth and treatment of the film were found to provide a high-quality positron moderator. The slow to fast positron conversion efficiency, using a Nan source, was found to be 4 X 10- 4. The use of moderated positron beams has become active since studies have shown that it is a useful surface and nearsurface probe. 1.2 An essential factor has been the development in moderator efficiency, which has increased by a factor of lOS since its discovery in the 1950's. J Searching for high conversion efficiency moderators is still an important task for the development of slow-positron physics. A typical laboratory positron beam begins with a positron-emitting radioactive source (i.e., C0 5S or Na 22 ) irradiating a well annealed metal moderator. This process is relatively inefficient as approximately 10- 3 are moderated and reemitted from the surface of the negative positron work function moderator. At the present time only a backscattering mode has been utilized, that is, where the positron enters and leaves from the same surface. This mode has been successfully used for both initial moderation of sources,2 and more recently for a brightness enhanced remoderated beam.4 One of the disadvantages of the back reemission moderator is that a significant fraction of the moderated positrons are recaptured by the source cross section when they drift away from the moderator, known as the "source shadow" problem. It is this problem that seriously restricts the present use of Na 22 sources which are commercially available only in large diameter configurations (;:::: 1.3 cm). Although this source has several advantages such as a :long half-life (2.6 yr), high (J + decay fraction (90%), and can now be made in a high specific activity form, the shadow problem is a serious obstac1e. A forward reemission geometry (W vane) has been used for a W moderator to produce a slow positron beam. s A second disadvantage present in initial moderation is that the phase space of the moderated beam is distorted by the electric fields and geometry associated with the backscattering mode. By fabricating a low defect concentration single-crystal thin film which has a large negative positron work function (> 1 eV) one can remove most of these constraints. In this letter we describe the first implementation and use ofa thin-film W (100) moderator. This moderator was tested with a Na 22 source. The thinfilm moderator was a 5000 A W (1 (0) film grown by high239

Appl. Phys. Lett. 47 (3). 1 August 1985

vacuum electron beam evaporation. The crucial technical step is to fabricate high quality [i.e., low defect (10- 6 at. fraction)] and low impurity concentration), cleanable, single-crystal metal thin films. A tungsten film was chosen because of its high mass density and because it has been reported 6 that W single crystals produce the highest conversion efficiency in the backscattering mode (;::::2 X 10- 3 ). The film growth technique employed was similar to Mertler et al.,7 and has been described by Chen et al. R Initially large mosaic spread (;:::: 10 deg) molybdenum single crystals were grown on carefully polished MgO (100) host substrates and this composite served as the substrate for the W film. The polished substrate was mounted on a Nb holder which maintained the substrate temperature at ;:::: l000·C while the Mo and W were deposited subsequently at 3-4 A/s onto the substrate. Base pressures were in the low 10- 8 Torr range and rose to 1.5x 10- 7 Torr during the W evaporation. To remove the self-supporting W films, 80% H 2S04 and 20 vol % H 2 0 solution at 90 ·C was used to dissolve the MgO crystal and the Mo layer was removed thereafter by etching in a solution consisting of 45 vol % H 2S04, 25 vol % HN0 3 , and 30 vol % H 2 0 at 70-90 ·C. Both x-ray and transmission electron microscopy measurements were done to confirm the orientation and the quality of the film. A 1 Xl cm W (100) film was obtained and inserted between a folded 95% transmission W mesh. The foil and mesh were affixed on a male-type bayonnet Ta mount. The mount and foil were heated in 10- 7 Torr oxygen for 20 min and flashed between 1800-2000·C at 10- 8 Torr three times before installation in the positron beam apparatus. From an earlier study8 we found this procedure to be adequate in removing defects and carbon impurities from the thin film and W mesh. The foil was inserted through the rear of the source chamber (Fig. 1) with the Na 22 source plug removed. The foil and mount are located inside a nonmagnetic W(90 wt. %) alloy. This W alloy is used for gamma-ray shielding and has been vacuum outgassed to remove trapped gases present after fabrication (i.e., hot pressing). The mount and foil are inserted on a long rod which has a reverse bayonnet lock. After the foil is in place, the grounded Na 22 source plug is

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© 1985 American Institute of Physics

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FIG. 1. Full source region of the positron beam apparatus is shown. The cross hatched regions delineate those places where the tungsten alloy is present for gamma-ray shielding. The W(IOO) foil is housed in the W alloy and the source is moved into place on the linear feedthrough. The electron gun is used to heat the foil with the source removed and guiding magnetic field run at approximately 70 G. The temperature of the foil is monitored during heating with an optical pyrometer. The full source chamber can be electrically floated up to 100 ke V.

installed through the same flange. The foil and mesh can be heated in situ by a movable electron gun shown in Fig. 1 and gases (i.e., oxygen) can be admitted by a leak valve not shown in the figure. During in situ heating the Nan is moved back on the linear motion and positioned in the second W alloy shielding shown in Fig. 1. A single set of E X B filters is used to remove any fast positrons that are transmitted through the foil along the magnetic field axis. The entire source chamber is electrically floating so that the positron energy can be varied between 0 and 100 keV. The large solenoid. coils (Fig. 1) produce a guiding magnetic field of 60-100 G. To attain an ultimate vacuum in the 10- 10 Torr region, the system is baked to approximately 135°C. The Na 22 is mounted on a linear feedthrough which can be moved to within 1 mm of the W( 100) foil. After baking the system, a moderator efficiency of :::: 3 X 10- 4 was determined with a calibrated Nal(TI) gamma-ray detector located at the target end of the chamber (not shown in the figure). After repeated annealing cycles (:::: 1500--1800 0c) and oxygen treatments, a moderator efficiency of only 4 X 10- 4 was found. It should be noted that source-to-moderator distance should be minimized to minimize the positron beam size.

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The present beam spot is found to be 7 mm in diameter, using a channel electron multiplier array coupled to a phosphorus screen. This measured efficiency is below the theoretical prediction 9 of 10- 3 , however, this may be associated with the specific activity of the Na22 ,10 and not the quality of the W(loo) foil. The present source and foil configuration has been operating for months with no degradation in the slow positron intensity. In conclusion, we have shown that thin W( 100) films can be made of a high enough quality (i.e., low defect concentrations) to be used as an initial positron moderator with a moderator efficiency of ::::Ax 10- 4 . The film does not poison easily and has been shown to produce a stable intensity for a period of a few months. It is suggested that these films should also work well in a low vacuum system (10- 6 Torr), thus reducing the cost of a relatively expensive ultrahigh vacuum source chamber. With the same foil installed in an electrostatic beam at Brookhaven, which presently houses a backscattering moderator, a further reduction in the beam diameter of 5 should be realized from simple phase space arguments. 4 We wish to thank D. M. Chen for help in removing the film and J. J. Hurst for x-ray measurements. Work performed at Brookhaven is supported by the Division of Materials Sciences, U.S. Department of Energy under contract DE-AC02-76CHOOO16. IA. P. Mills, Jr., Science 218,335 (1982). 2K. G. Lynn, in Positron Solid State Physics. edited by W. Brandt and A. Dupasquier (North-Holland, New York, 1983), pp. 609-643; A. P. Mills, Jr., ibid., pp. 432-509. 'w. Cherry, Ph.D. dissertation, Princeton University 1958, available from University Microfilms, Inc., Ann Arbor, Michigan. ·W. E. Frieze, D. W. Gidley, and K. G. Lynn, Phys. Rev. B31, 5628 (1985). 'P. W. Zitzewitz, T. C. Van House. A. Rich, and D. W. Gidley, Phys. Rev. Lett. 43,1281 (1977). 'A. Vehanen, K. G. Lynn, P. J. Schultz, and M. Eldrup, Appl. Phys. A 32, 2572 (1983). 7G. Merder, M. Rey, and K. Reichett, Nucl. lnstrum. Methods 192, 535 (1982). "D. M. Chen, K. G. Lynn, R. Pareja, and Bent Nielsen, Phys. Rev. B 31, 4123 (1985). 9A. Vehanen and J. Makinen, Appl. Phys. 36, 1(1985). "'The Nan source was purchased from New England Nuclear. The capsule is 1/2" in diameter with a Be backing (used in spin polarized positron studies) and a 5-mil Ti foil to prevent the source leakage in the vacuum. The active diameter of the Nan source is 0.125'. The vendor downgraded the source strength owing to problems with solid waste mixed in with the Nan acetate. This is atypical however, it causes concern in knowing the specific activity which is estimated to be less than 200 Ci/g.

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