Correlated EPR and ionic thermocurrent (ITC) measurements of CaF2, SrF"and BaF, containing trivalent gadolinium have allowed tentative identification of two types of Gd'+-fluoride interstitial (F, ) complexes in each fluoride and have determined the respective reorientation kinetics and relative concentrations of each complex. The type-I complex has C4"symmetry with the F, in a nearest-neighbor site. The type-II complex has C"symmetry with the F, . in a next-nearest-neighbor site. The ratios of concentrations of type-I to type-II complexes in the crystals as grown were found to be about 10 in CaF2:Gd + (0.1 mole%), about 4 in SrF2:Gd + (0.1 mole%), and about 0.1 in BaF2. Gd'+ (0.05 mole%). The temperature of the type-I ITC peak increased while the temperature of the type-II peak decreased in going from CaF, to SrF, to BaF,. The activation energies E for reorientation of the type-I complex were 0.42, 0.45, and 0.46 eV for CaF"SrF, and BaF2, respectively.For the type-II complex in CaF&, SrF&, and BaF&, E w» 0.69, 0.62, and 0.60 eV, respectively. For the type-II complex the reciprocal frequency factor To was of the order of 10 " sec in all three fluorides. For the type-I complex To was 2.5 X 10 ', 1 X 10 '", and 1 X 10 ' sec in CaF&, SrF&, and BaF&, respectively.

Ionic thermocurrent (ITC) measurements have been used to determine the relaxation parameters for the principal (type-I) dipolar complex in CaF, :R'+, where R is used here to represent Sc, Yb, Tm, Er, Y, Dy, Gd, Eu, Nd, Ce, and U. An activation energy (E) of about 0.38 eV and a reciprocal frequency factor (To) of about 2 X 10 ' sec was measured for Sc'+, the smallest impurity ion measured. For the five largest impurities studies, E was about 0.42 eV and To was about 3 &( 10 " sec. A uniform transition between these two sets of relaxation parameters was observed over a relatively small range of R'+ ionic radii. These results are compared with other recently reported data and possible models for the observed behavior are discussed.

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The effects of the mammography film processing replenishment rate on contrast and speed are studied sensitometrically. Two experiments studied decreasing replenishment rates in the Kodak RP developer and quantified changes in the developer by measuring bromide ion concentrations. First, values of NaBr concentration from 1.7 to 8.4 g/L, achieved by reducing the replenishment rate, were tested with sensitometry strips. Second, the developer replenishment rate of a high volume dedicated mammography processor was reduced by one-third, to 20 cm3/1560 cm2, so that the NaBr concentration rose from 2.0 to 12.36. Sensitometric results for four film types and patient films were tested for changes from standard values as NaBr concentration was restored to 3.31 g/L. Fifty-five clinical images obtained at 7.3-9.3 NaBr g/L were compared to their matching previous films, with NaBr levels of 2-3 g/L, for contrast and visibility of the skin line. For the range of the NaBr ion from 1.7 to 7 g/L, no significant sensitometric differences were found. Above 7 g/L, different film types had different sensitometric results. From 7.3 to 9.3 NaBr g/L, 47.5% of the clinical films reviewed by four radiologists had less contrast compared to previous films. Dedicated mammography processors with high film volume (i.e., those that do not have excessive oxidation or foreign dye problems) can operate at lower replenishment rates than are currently employed. All common mammography film types are stable at these lower replenishment rates up to 7.0 NaBr g/L.