Solid Kr moderator for producing slow positronsA. P. Mills Jr., S. S. Voris Jr., and Tamara S. Andrew Citation: Journal of Applied Physics 76, 2556 (1994); doi: 10.1063/1.357550 View online: http://dx.doi.org/10.1063/1.357550 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/76/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Slow positrons elastically scattered by solid targets J. Appl. Phys. 96, 807 (2004); 10.1063/1.1739289 Enhanced slow positron reemission with new thin foil moderator geometry Appl. Phys. Lett. 57, 998 (1990); 10.1063/1.103536 Slowing down of positrons in solids AIP Conf. Proc. 205, 639 (1990); 10.1063/1.39239 Solid neon moderator for producing slow positrons Appl. Phys. Lett. 49, 1121 (1986); 10.1063/1.97441 Studies of the formation of slow positrons in MgOcoated moderators J. Appl. Phys. 51, 856 (1980); 10.1063/1.327658

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Sollid Kr moderator for producing slow ositrons A. P. Mills, Jr., S. S. Voris, Jr., and Tamara S. Andrew@ AT&T Bell Laboratories, Murray Hill, New Jersey 07974

(Received 11 March 1994; accepted for publication 9 May 1994)

We find that proper annealing of a solid Kr positron moderator results in a fast positron to slow positron conversion efficiency 90% as good as that of solid Ne. Our results are in agreement with Grund, Maier, and Seeger [Mater. Sci. Forum 105-110, 1879 (1992)] except that our absolute efficiencies (~0.2%) are higher presumably due to the better vacuum conditions of our experiment. Since the Kr does not have to be cooled below 10 K, the moderator can be made using an inexpensive refrigerator and is suitable for a large surface area intense positron source.

Slow positrons are eV energy positive electrons pro- duced by moderating the =MeV positrons one obtains from /3+ decay radioactivity or pair-production.“’ Slow positrons are important because they may be easily focused and trans- ported as a beam for analyzing materials and for atomic physics studies. Possible solid moderators include metals such as W, Cu, Ni, and nonmetals such as diamond and the rare gas solids. The metals and diamond3 have a negative affinity for positrons. Energetic positrons stop in a distance X of order 15 pm in W and diffuse at thermal energies L - 0.1 pm or more. Thus roughly 1% of the e+ from a radioactive source such as “Na encounter the moderator surface where there is a ~-30% chance that the positrons will escape into the vacuum because of their negative affinity. The efficiency for converting source positrons to slow positrons is e=gLyl X, which can be as much as 0.3% (Ref. 4) depending on geometrical factors g.

The rare gas solids5 are good moderators because after stopping in the solid, i.e., after the positron kinetic energy is no longer sufficient to cause ionization, the positrons lose energy only due to phonon emission, retaining several eV of kinetic energy while diffusing a distance L of several pm. Upon encountering the rare gas solid surface, some of the hot positrons are able to escape into the vacuum despite their positive affinity 4+ for the solid if their kinetic energy pro- jected perpendicular to the surface exceeds ++. A solid Ne moderator’ has thus far shown the highest fast positron-to slow positron conversion efficiency of any moderator.

At the time of the invention of the solid Ne.moderator, the other rare gas solids were tried as well, but were reported to have efficiencies only one-fifth of the Ne moderator. More recently, Grund, Maier, and Seeger7 reported that K.r and Xe had efficiencies nearly as good as that of Ne, although the absolute efficiencies were only one-eighth of the e=O.7% original solid Ne moderator.6 One possible explanation that dissuaded us from exploring the result of Ref. 7 further was that the deliberately chosen poor vacuum environment (10e6 Torr) of Ref. 7 was reducing the Ne moderator efficiency much more than that of Kr and Xe. The true significance of Ref. 7 actually has to do with their annealing of the rare gas solids. At the insistence of Eberle and Hulett’ we have rein- vestigated the rare gas solid moderators in our ultrahigh vacuum positron beam designed for Ne moderation of a

“‘Current address: Physics Dept., Brandeis University, Waltham, MA 02254.

“Na source. Our new result confirms the relative efficiencies of Ref. 7 for properly annealed rare gas solid moderators, while achieving high absolute efficiencies.

In retrospect, we now know the reason for the poor ef- ficiency for Kr and Xe in the experiment of Ref. 6 was that a high enough annealing temperature could not be reached be- cause of the detrimentally high vapor pressure of gas evapo- rating from parts that were initially colder than the radioac- tive source, but which became warmer than it during the annealing process. The present design does not suffer from such problems and includes a heater and Si diode thermom- eter in contact with the source holder. The source was ‘“Na deposited on a 7.6-mm-diam spot and covered with a 5 pm CuBe foil. The foil is sealed by an In gasket pressed on its top surface. At the time of the measurements the source strength was (8025) mCi determined from the intensity of the 1.274 MeV nuclear gamma ray measured at a distance of 7 m with only a vacuum viewport and the air attenuating the photons. The source holder is a cylinder of 80% W, 20% Cu material 31.8 mm in diameter and 50.8 mm long with a cover of the same diameter and 25.4 mm in length of the same material. The cover has a slightly paraboloid conical hole 7.6 mm in diameter at its base where it presses against the In gasket and open up to 15.2 mm in diameter at the exit face. Comparison of the temperature reading of a Si diode on a dummy source with the cold end of the refrigerator shows that the source holder is less than 0.5 K warmer than the thermometer on the refrigerator at temperatures as low as 4 K.

The research grade purity rare gases used were further purified by means of an all stainless steel getter furnace con- taining Ti at a temperature of 800 “C. The rare gas modera- tors condense on the inner surface of the source cone from a supply tube 3.2 mm o.d. of 304 stainless steel, the orifice of which is 1 cm from the edge of the cone. The source holder is screwed onto the 19 mm diameter by 19 mm long end of a 4 K closed cycle refrigerator using a single l/4-28 screw and an In gasket for reduced thermal impedance. The ther- mometer is a Lake Shore Si diode specified to have to.25 K accuracy in the 2-100 K range. The diode is potted with low temperature compatible epoxy inside a hole in the 19-mm- diam refrigerator cold end. The heater is wound nearby on the same Cu cylinder. Surrounding the source holder is a -70 K cold shield that is plugged by a series of 12.5 mm inside diameter baffles at a temperature of ~150 K. The

2556 J. Appl. Phys. 76 (4), 15 August 1994 0021-8979/94/76(4)12558/3/$6.00 @ 1994 American Institute? of Physics

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oL,*L,-,' * 'I ) b 8 1' 3 " I 0 100 200 300 400

f Cm)

FIG. 1. Count rate vs time while depositing Kr on a source at the indicated temperatures.

source vacuum chamber base pressure was 1.0X 10m9 Torr after bake-out. The refrigerator had a small He leak that caused the base pressure to rise to 1.0X10-* Torr when it was cold.

Slow positrons from the moderator pass through an EX B velocity filter (Wein filter) and strike a Ni target biased at -900 V. The entire refrigerator is floating relative to the rest of the vacuum can at the desired positron beam energy, +30 V. Annihilation photons from the target are detected by a 3X3 in. NaI(Tl) scintillation detector located with its center 650 mm from the center of the target in a line that is at right angles to the positron beam axis. The calculated efficiency for detecting positrons is 11720, including an average solid angle dflJ2rr= l/583 for the two annihilation photons, the 0.90 efficiency for detecting a 511 keV annihilation photon, and a 90% transmission probability due to the vacuum win- dow separating the target from the detector. The total posi- tron emission rate of the source is (2.68+0.17)X109 e+s-‘. The moderator efficiency is thus given.by the NaI(Tl) count rate times (2.69+O.17)X1O-7 s.

The results for the NaI(T1) count rate versus time as Kr is being deposited at different source temperatures is shown in Fig. 1. The pressure in the vacuum container at a point removed from the gas outlet was 3 X lop5 Torr for the dura- tion of the deposition. Since the curves are not simple expo- nential functions of time, the gas deposition may be occur- ring with different rates and the moderation efficiency may not vary with thickness in the same way on different parts of the source cone. Deposition of the Kr at 4.5 K results in a moderator that is inferior to those deposited at 25 K or higher temperature. Possibly the Kr condensed at 4.5 K has a high density of defects that results in a short diffusion length for eV kinetic energy positrons. Since elastic positron-atom col- lisions are the only energy loss mechanisms, the total path length of the epithermal positrons S=NA is constant, N be- ing the number of collisions and A the mean free path per collision. On the other hand, the diffusion length is L = fiA, assuming the elastic collisions occur with signifi-

8000 a 3,. 8, a a , , , c , , , , , I , ,

ANNEALING OF SOLID Kr MODERATORS * t

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FIG. 2. Count rate while subjecting the Kr moderators of Fig. 1 to tempera- ture cycling.

cant momentum transfer on the average. Thus, the diffusion length and hence the moderator efficiency, is proportional to 6. Solid Kr that is granular on a scale smaller than the 100 8, mean free path for phonon emission in the crystalline solid will be a poor moderator.

The NaI(T1) count rate as the moderators of Fig. 1 are annealed is displayed in Fig. 2. The arrows indicate the time progression. The moderators deposited at 4.5 and 26 K im- prove significantly after warming to 43 K and cooling to 3.5 K. The 36 K deposited moderator shows no annealing effect, i.e., no temperature cycling hysteresis, but both the 36 and 43 K deposited moderators work better at lower tempera- tures. Although there seems to be a trend towards better ef- ficiency after annealing and cooling associated with higher deposition temperature, the difference may not be significant for the 26, 36, and 43 K moderators. The 4.5 K moderator is worse than the others after annealing, but this is likely due to the deposition having been terminated after 150 m compared to more than twice the exposure at the other temperatures.

The energy spread of the Kr moderator was found by measuring the NaI(T1) count rate as the bias on the baffles in front of the source was varied. The longitudinal component of the energy distribution shown in Fig. 3 has a 10% to 90% integral width of (4.220.2) eV.

A comparison of the Kr moderator with Ne and Xe as well as their growth and decay is shown in Fig. 4. The Ne moderator is initially the most efficient, but it decays rapidly to become less efficient than Kr. Ne showed no improvement upon annealing. Xe was a little worse than Kr initially and rapidly decayed ibefficiency, possibly because of an electron irradiation effect.’ After one day the Xe moderator recovered by annealing to almost its original efficiency, but then de- cayed again, so the decay is not principally due to removal of Xe from the source holder. The Kr curve is a composite of two runs, so the apparent drop in yield immediately after annealing is fictitious. Solid Kr is the best moderator for efficiency after about 6 h and remains relatively constant for about one day. The best absolute efficiencies represented by

J. Appl. Phys., Vol. 76, No. 4, 15 August 1994 Mills, Jr., Voris, Jr., and Andrew 2557 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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4 - 2 0.8 - ‘1 - :: - z 2 0.6 - t: _

_ g - p 0.4 - Ei _ ” - a - E 0.2 -

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Qt.-----

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i :A., :. -‘ *. 5.~ i. 4

‘; z

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‘“~“““““““,‘$ GROWTH AND DECAY OF~SOLID RARE GAS MODERATORS t &+z

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FIG. 3. Integral distribution of the normal component of the positron ener- gies emitted from a Kr moderator. The slight oscillation is due to a nonlinear digital to analog converter.

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the count rates in Fig. 3 are Ne: (0.23+0.02)%; Kr: (0.21 t0.02)%; and Xe: (0.16+0.02)‘%. The stated errors reflect our estimates of the calibration and statistical uncertainties.

While the dependence on the source strength, vacuum contaminants, and deposition rate of the decay effects and efficiency would be interesting to study in detail, it is impor- tant to note that the data presented in Figs. l-4 may be dependent on the precise configuration of the apparatus and should not be expected to be reproduced in detail by other experiments. Nevertheless, our conclusion is not sensitive to such detail and is in agreement with Ref. 7: solid Kr is an easily fabricated and highly efficient slow positron modera- tor that requires only a single stage 10 K refrigerator. Having been hampered by the 4 K temperatures needed for solid Nel

FIG. 4. Comparison of solid Ne, Kr, and Xe rare gas moderators. The total efficiency for converting source positrons to slow positrons is given by the NaI(TI) count rate times the factor 2.7X10-’ s.

the rare gas moderator will,now be more widely useful. The authors would like to thank Chris Eberle and Lester

Hulett for encouraging us to perform the measurements on solid Kr and Karl Canter for useful discussions.

IA. P. Mills, Jr., in Positron Solid Stare Physics, edited by W. Brandt and A. Dupasquier (North-Holland, Amsterdam, 1983), p. 432.

‘P. J. Schultz and K. G. Lynn, Rev. Mod. Phys. 60, 701 (1988). 3G. R. Brandes, A. P. Mills, Jr., and D. M. Zuckerman, Mater. Sci. Forum

LOSHO,1363 (1992). 4A. Vehanen, K. G. Lynn, P. J. Schultz, and M. Eldrup, Appl. Phys. A32,

163 (1983). “E. M. Gullikson and k P. MiIls, Jr., Phys. Rev. Lett. 57, 376 (1986). ‘A. P. Mills, Jr. and E. M. Gullikson, Appl. Phys. Lett. 49, 1121 (1986). ‘T. Grund, K. Maier, and A. Seeger, Mater. Sci. Forum 10.5-110, 1879

(1992). “C. Eberle and L. D. Hulett (private communication). 9H. H. Farrell, Myron Strongin, and J. M. Dickey, Phys. Rev. B 6, 4703

(1972).

2558 J. Appl. Phys., Vol. 76, No. 4, 15 August 1994 Mills, Jr., Voris, Jr., and Andrew

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