Robert K. Smither

Liquid gallium cooling of silicon crystals in high intensity photon beams (invited)

The high‐brilliance, insertion‐device‐based photon beams of the next generation of synchrotron sources (Argonne’s APS and Grenoble’s ESRF) will deliver large thermal loads (1–10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and various cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in UHV conditions. A series of experiment...

New method for focusing x rays and gamma rays

A new method for focusing x rays and gamma rays is described that can focus monochromatic radiation from a point source or parallel beam down to a line image. If two focusing elements are used or if a single element bent in two directions is used, the radiation can be focused to a point image. Conversely, radiation from a point source can be converted into a parallel beam. The method makes use of bent diffraction crystals in which the intercrystalline‐plane spacing is varied as a function of position in the crystal. The Bragg angle for diffraction of monochromatic radiation will now vary as a function of position in the crystal, and this new degree of freedom can be used to obtain focusing of the diffracted beam. A number of ways to achieve this variation in crystal‐plane spacing is discussed, including the use of thermal gradients and the variation of the elemental composition of the crystal. The applications of this new focusing system to a gamma‐ray telescope and to the production of a real image of a ...

Half-life of 60Fe

Abstract The half-life of 60Fe has been measured to be T 1 2 = (1.49 ± 0.27) × 10 6 a, significantly longer than the one previous measurement of Roy and Kohman which reported a value of 3 × 105 a uncertain by a factor of 3. The present value was obtained from measurements of specific activity and radioisotope concentration in a material produced by spallation of copper with 191 MeV protons. 60Fe/Fe ratios in the range of 10−8 were measured with the Argonne FN tandem-superconducting linear accelerator system in conjunction with an Enge split-pole spectrograph. The specific activity of 60Fe in Fe was measured through the grow-in of the 1.332 MeV gamma-ray line of the 60Co daughter activity.

Liquid gallium metal cooling for optical elements with high heat loads

Abstract The intense photon beams from the insertion devices of the Argonne Advanced Photon Source (APS) will have very high total powers, which in some cases will exceed 10 kW, spread over a few cm 2 . These high heat loads will require special cooling methods for the optical elements to preserve the quality of the photon beam. A set of finite element analysis calculations were made in three dimensions to determine the temperature distributions and thermal stresses in a single crystal of silicon with heat loads of 2–20 kW. Different geometric arrangements and different cooling fluids (water, gallium, oil, Na, etc.) were considered. These data were then used in a second set of calculations to determine the distortion of the surface of the crystal and the change in the crystal plane spacing for different parts of the surface. The best heat transfer, smallest surface distortions and smallest temperature gradients on the surface of the crystals were obtained when the cooling fluid was allowed to flow through channels in the crystal. The two best fluids for room temperature operation were found to be water and liquid gallium metal. In all cases tried, the variation in temperature across the face of the crystal and the distortion of the surface was at least a factor of two less for the gallium cooling case than for the water cooling case. The water cooling was effective only for very high flow rates. These high flow rates can cause vibrations in the diffraction crystal and in its mount that can seriously degrade the quality of the diffracted photon beam. When the flow rates were decreased the gallium cooling became 3–10 times more effective. This very efficient cooling and the very low vapor pressure for liquid gallium (less than 10 −12 Torr at 100°C) make liquid gallium a very attractive cooling fluid for high vacuum synchrotron applications. A small electromagnetic induction pump for liquid Ga was built to test this cooling method. A pumping volume of 100 cm 3 /s was achieved. With no flow, a static head pressure of 8 psi was measured across the pump. With this flow rate of 100 cm 3 /s and a ΔT = 10° C , the heat transfer would be 2.2 kW of power. This system worked well and based on the test data from this system, a second electromagnetic induction pump was built that developed four times the flow rate. The new system is portable, controls the output temperature of the Ga and can handle heat loads of 10 kW. This work was supported by U.S. Department of Energy, BES-Material Science under Contract No. W-31-109-ENG-38.

MAX: a gamma-ray lens for nuclear astrophysics

The mission concept MAX is a space borne crystal diffraction telescope, featuring a broad-band Laue lens optimized for the observation of compact sources in two wide energy bands of high astrophysical relevance. For the first time in this domain, gamma-rays will be focused from the large collecting area of a crystal diffraction lens onto a very small detector volume. As a consequence, the background noise is extremely low, making possible unprecedented sensitivities. The primary scientific objective of MAX is the study of type Ia supernovae by measuring intensities, shifts and shapes of their nuclear gamma-ray lines. When finally understood and calibrated, these profoundly radioactive events will be crucial in measuring the size, shape, and age of the Universe. Observing the radioactivities from a substantial sample of supernovae and novae will significantly improve our understanding of explosive nucleosynthesis. Moreover, the sensitive gamma-ray line spectroscopy performed with MAX is expected to clarify the nature of galactic microquasars (e+e- annihilation radiation from the jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and, last but not least, gamma-ray afterglow from gamma-burst counterparts.

A virtual phase CCD detector for synchrotron radiation applications

A two‐dimensional charge coupled device (CCD) detector, based on the Texas Instruments ‘‘virtual phase’’ CCD, has been developed for synchrotron radiation applications. Simultaneous near‐edge and multilayer scattering experiments have been carried out with the detector on an energy‐dispersive synchrotron beamline. The detector was used in an optical mode where the CCD element is coupled to a phosphor screen by a pair of focusing and demagnifying lenses. We report on the performance of the detector in this mode.

Level scheme of Au198 determined by analysis of high-precision capture gamma-ray measurements☆

Abstract The gamma-ray spectrum resulting from the capture of thermal neutrons by Au 197 has been investigated by use of the Argonne 7.7-meter bent-crystal spectrometer. A total of 122 lines corresponding to transition energies less than 835 kev were observed. Their energies were determined with an average precision of 1 part in 5000. The method of generating a “most probable” level scheme for Au 198 from these measurements is described. A scheme containing 25 states is obtained which shows an unusual “fine structure” grouping of several levels.

Performance of a gallium-cooled 85° inclined silicon monochromator for a high power density X-ray beam

We have made double crstal rocking curve measurements on a gallium-cooled silicon monochromator in both the normal flat geometry and an 85° inclined geometry on the X-25 focused wiggler beamline at the National Synchrotron Light Source. At 192 mA ring current, the focused wiggler delivers about 37.7 W of power into a spot size of FWHM 0.4 × 0.8 mm2, resulting in an average power density of about 118 W/mm2. The inclined crystal geometry spreads the beam footprint on the surface of the crystal while maintaining a b = −1 symmetric Bragg reflection. At an 85° inclination angle, the beam footprint is 11.5 times larger than that for the flat geometry. In the case of the flat geometry at a ring current of 156 mA, we see, via an infrared camera, an increase in temperature of 56°C above the nominal silicon temperature. The rocking curve this case were significantly broadened (FWHM for 15 keV Si(333) = 35 arcsec) due to the thermally induced strain in the silicon. In the inclined crystal, the thermal peak on the crystal was only about 2.7°C above the nominal silicon temperature. In this case, the rocking curve width for the 15 keV Si(333) reflection was measured to be FWHM = 2.7 arc sec compared with the theoretical width of FWHM = 1.0 arcsec. The residual strain is totally due to the mounting of the crystals and not the heating from the X-ray beam.

Summary of a workshop on high heat load X-ray optics held at argonne national laboratory☆

Abstract A workshop on High Heat Load X-Ray Optics was held at Argonne National Laboratory on August 3–5, 1989. The workshop was co-sponsored by the Advanced Photon Source and the European Synchrotron Radiation Facility and served as a satellite conference to SR189. The object of this workshop was to discuss recent advances in the art of cooling X-ray optics subject to high heat loads from synchrotron beams. The cooling of the first optical element in the intense photon beams that will be produced in the next generation of synchrotron sources is recognized as one of the major challenges that must be faced before one will be able to use these very intense beams. Considerable advances have been made in this art during the last few years, but much work remains to be done before the heating problem can be said to be completely solved. Special emphasis was placed on recent cooling experiments and detailed “finite-element” and “finite-difference” calculations comparing experiment with theory and extending theory to optimize performance. Copies of the Proceedings can be obtained from B. Meyer, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.

Measurement of the 27Al(n,2n)26Al reaction cross section for fusion reactor applications

The 27Al(n,2n)26Al reaction is of considerable interest to the fusion reactor program. Aluminum is an attractive material for many structural applications and the (n,2n) reaction is the major source of long-lived activity (26Al, g.s. T12 = 7.34 × 105 y). The threshold for this reaction falls within the spread of neutron energies generated by a D-T plasma. Its cross section is therefore a steeply rising function of energy for the primary fusion neutrons. This special feature makes it possible to use this reaction to measure plasma ion temperatures as well as neutron yields and neutron spectral shapes. The above applications require accurate measurements of the 27Al(n,2n)26Al cross section near threshold, but no data was available in the 14–15 MeV energy range of interest so a series of experiments1,2 were performed using aluminum samples irradiated at the neutron sources at LLL (RTNS-II), 3,4 and at PPL (electrostatic generator).2 In some experiments the new technique of accelerator mass spectrometry1,5 was used to measure the production rate of 26A1. The new measurements1 suggest that most of the (n,2n) yield near threshold is associated with the direct production of the 3+ state of 416.9 keV rather than the 5+ ground state. This raises the Q-value and neutron threshold energy for this reaction by a similar amount and greatly reduces the effective cross section of the (n,2n) reaction for D-T fusion neutrons. The effective cross section for a 9 keV D-T plasma is reduced by a factor of 5 by this shift in Q-value. A similar reduction occurs for the long-lived radioactivity.