Many cold positrons in ultrahigh vacuum are required to produce cold antihydrogen, to cool highly stripped ions, and for ultracold plasma studies. Up to 3.5 3 10 4 such positrons have now been accumulated into the ultrahigh vacuum of a 4.2 K Penning trap, at a rate exceeding 10 3 ͞h. Both the accumulation rate (per high energy positron incident at the trap) and the number accumulated are much larger than ever before realized at low temperatures in high vacuum. The cooling of high energy positrons (from 22 Na decay) in a tungsten crystal near the trap, together with purely electronic trapping and damping, is the key to the efficient accumulation and to projected improvements.

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Coherent and incoherent scattering cross sections have been measured using a high purity germanium detector on elements in the range of Z = 13–50 using 241Am gamma rays. The cross sections have been derived by comparing the net count rate obtained from the Compton peak of aluminum with the elastic/Compton peak of the target element. The measured cross sections for the coherent and incoherent processes are in agreement with the theoretical values.

1)3)-Introduction Studies of CdS thin films by using different techniques /1 to 5/ have gained importance recently because these films a r e used as window material in CdS/CuxS solar cells. Investigations have generally focused on the effect of thickness of films on the grain size. Studies on the effect of temperature /6,7/ have been rare. Also the research in this field has taken a newpath towards the possibility of using very thin CdS layers instead of thick ones ( e l 0 to 15 Mm) for the sake of production of cheaper cells /8/. But the use of thin films is hampered by their low efficiency a s well a s degradation caused by surface effects and grain boundaries. In the present note, this problem is investigated in the case of very thin CdS films. The relation between surface nature and the temperature of deposition of films is studied in this work.Theory The size factor of surface irregularities is calculated from the refractive index variation following the technique of Azzam and Bashara /9/ a s reported earlier /lo/. Here it is assumed that the film i s having a two-layer structure; a rough upper layer with its thickness equal to the r. m. s. value of the height of the irregularities and a lower layer with a perfectly smooth surface. Due to the presence of the rough upper layer, the measured value of the refractive index (N ) will be different from the actual value (Nf) of the material of the film. The relation between these two is expressed as /11/ e e ' 2 -= 9 7 9 N e + 2 N f + 2 where q is the volume factor of the surface irregularities.

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