The low-temperature X-ray diffraction studies of orthorhombic insulin citrate crystals reported here were initially undertaken because of the rapid deterioration at room temperature of certain insulin crystals containing heavy-metal cations. The effects of cooling on mosaic character and radiation sensitivity were studied. Two temperature ranges were employed: (a) below -150'C and (b) 00C to -13'C.In normal laboratory practice orthorhombic insulin citrate crystals are grown and stored at 1 d 10C. Immersion studies in heavy-atom reagents are also made at this temperature. All X-ray diffraction work is, however, carried out at room temperature (.210C).We have found that orthorhombic insulin citrate crystals may be cooled rapidly to below -150'C without impairing the X-ray diffraction pattern. Rapid cooling does, however, enhance the mosaic character of these crystals. X-ray diffraction studies at 00C and -13'C have provided evidence for marked reduction in radiation sensitivity in certain heavy-atom-containing crystals.General Background.-Although metal-free insulin crystals are somewhat more sensitive to radiation damage (nickel-filtered CuKa) at room temperature than at 00C, the radiation sensitivity at room temperature is not marked enough to prevent the collection of intensity data by counter techniques. ' Furthermore, these crystals do not deteriorate on standing at room temperature for several weeks without irradiation.

Measured values of the relative intensities of several K-fluorescent x rays are reported for 28 elements ranging from calcium through plutonium.Solid-state Si(I i) and Ge(Li) spectrometers and a Bragg-diffraction spectrometer were used in making the measurements.Fluorescence was excited in thin samples by higher-energy x rays. Corrections were made to the measured relative intensities for sample absorption, air absorption, and spectrometer efficiency. The accuracy of most of the measurements is +2 to 5%. The results are compared with those of other experimenters and with the Scofield calculations. 1NTRODUCTION Several recent experimental determinations of relative intensities of K emission lines have been reported. ' Most of these papers describe measurements made for a relatively small number of characteristic x rays or for relatively few elements.The purpose of the present work is to provide experimental measurements of the intensities of several K emission lines relative to the Ko'& intensity for a large number of elements from Z = 20 through 2= 94. Thin samples were used as fluorescers so that corrections for x-ray absorption would be minimized. Three spectrometers were used in these measurements.A Bragg-diffraction spectrometer with high-resolution Soller slits was used to measure K&2-to-K&& ratios for x-ray energies less than 50 keV; a Si(Li) spectrometer was used to measure Kp-to-Kn ratios for x-ray energies less than 20 keV; and a Ge(Li} spectrometer was used to make all of the higher-energy measurements.Peak

Measured values of the relative intensities of several L fluorescent x rays are reported for 13 elements ranging from cerium through plutonium.A solid-state Si{Li) spectrometer and a Bragg-diffraction spectrometer were used to analyze the x-ray excited fluorescence from thin samples. Corrections were made to the measured relative intensities for sample absorption, air absorption, and spectrometer efficiency. The accuracy of the measurements varies from +5 to +15%. The results are compared with those from the Scofield calculations.

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