A study has been made of the ionization within a fiat cavity chamber under irradiation by X -and gamma rays in the energy region 38 to 1,250 kilovolts effectiv e (kev). Chamber walls were mad e of carbon, aluminum, copper, tin, and lead, and the wall separation was varied from 0.5 to 10 millimeters. Results are compared with cavity theory.

A discussion is presented of the practical considerations involved in wavelength-scanning polarization-modulation ellipsometry. Emphasis is placed on factors affecting accuracy and precision and on the alignment of the optical elements. The system described is used to measure the optical properties of air-cleaved KCI and of clean and tarnished Ag surfaces in ultrahigh vacuum in the 250-650-nm range.

A description of the new N a t ional Bureau of Standards " low" en ergy fr ee-air chamber is given. The sta ndard chamber is designed to mea sure t l1e expos ure dose II: roe ntgens for X-ray beams gen erated at pote ntials fr ol11. 20 to 100 kIi ovolts-co nstant-poten tJaI (kvcp) '':Ith filtration s ranging from 2 millimet e rs of b ery lli um to 2 mIi lune ters of b ery l hum plus 41m 11Ime ters of aluminum. The chamber ha s been compared wIth the N a tIOnal Bureau of Standards " medium" energy standard a t 60, 75, and 100 kvcp w ith filtration s of 8 , 3, and 4 millime ters of a luminum, r esp ectively. The t \\•o stand a rd cha mbers agreed to \\"lth ln 0.3 p erce nt.

No abstract

,-ray emission of radi um (roentgen s per m illi gram-hour at 1 centimeter) have been ma de by m eans of a graphite cavity ionization c ha mber. An au xi liary experi mental examination of th e stopping-power correction fo r t he non-air-equi:a lcn ce of.the cavity wall is a lso described . The r esult ing average valu e fot' t he -y-ray emlSSlOn of radIUm (enc losed in an 0.5-millimet er platinum capsule) is 8.26 ± 0.05 roentgen s p er mi ll igra m-hollr at 1 centime ter, whi ch is based upon the concluslOnt.hat the stoppmg-power data of Bakkcr and SegJ'c are con sistent with t he res ults of the a uxliJary expenment.