Empirical efficiency and volume relationships for HPGe detectors

We have extended the empirical work of Vano et al.[1] relating the slope of the detector efficiency curve to the active volume for Ge detectors. The analysis was carried out using Monte Carlo techniques and covered a wide range of incident energies (200 keV-20 MeV) and active volumes (19.6 cm³–396 cm³). It is shown that the expression of Vano et al.[1] is only valid over the energy range 200 keV-3 MeV for active volumes <50 cm³. The upper bound decreases to 2 MeV for volumes of a few hundred cm³. The usable energy range can, however, be extended to 6 MeV by introducing higher order terms into the polynomial. Above this energy, the shape of the efficiency curve is better described by a non-linear function since linear forms fail simultaneously to fit large active volumes and high energies. We therefore propose a composite function which reduces to the form given in Vano et al. in the low energy/active volume limit. By comparison with the Monte Carlo results, it is estimated that relative efficiencies can be calculated to within 6% over the energy range 200 keV-20 MeV and active volumes 20 cm³–400 cm³. Since the largest errors occur for the smallest volumes, we recommend that for energies <3 MeV a two-fold approach be followed, i.e. using the expression of Vano et al.[1] for active volumes less than 50 cm³ and the proposed non-linear form for larger volumes. For high energy work (E > 3 MeV), we advocate the non-linear form. In this way, average errors can be kept 3%. Finally, we point out that the real power of the expression of Vano et al. lies not in predicting efficiencies, but active volumes.     

Electrical pumping of color center lasers

This paper presents an assessment of the feasibility of using electrical pumping to achieve laser action in color centers in the alkali halides. We discuss mainly the self-trapped exciton in the alkali halides, which is a prime candidate for such a pumping scheme because it is known to produce strong electroluminescence. The theory of electrical contacts to insulating crystals is reviewed, as is the theory of space-charge-limited currents in insulators. It is shown that sufficient energy can be delivered to an alkali halide crystal by a space-charge-limited current to reach the threshold for laser action. We present experimental evidence which demonstrates that when ohmic contact is made to a KI single crystal, large space-charge-limited currents can flow. Finally, from the measured value of the electroluminescent intensity we infer the value of the population inversion achieved through electrical pumping of KI with blocking contacts.     

Calcium supplementation increases bone density in adolescent girls

In a longitudinal calcium intervention study, bone density was assessed in pubertal girls for 18 months. Significant additional increases in total body bone mineral density (1.3%) and spine bone mineral density (2.9%) and content (4.7%) were noted in the calcium-supplemented group. Increasing bone mass during adolescence with adequate calcium intake, if maintained into adulthood, could decrease the risk of osteoporosis later in life.     

How to model and simulate microgyroscope systems

An accurate physical model of a micro-electromechanical system can save valuable design time and slash the number of foundry runs needed for characterization. Using a set of existing simulators and field solvers, the authors developed a method for modeling the mechanical and electrical properties of their microgyroscope     

Dietary calcium, protein, and phosphorus are related to bone mineral density and content in young women

In aggressive interactions, animals often use a discrete set of signals, while the properties being signalled are likely to be continuous, for example fighting ability or value of victory. Here we investigate a particular model of fighting that allows for conventional signalling of subjective resource value to occur. Perfect signalling and no signalling are not evolutionarily stable strategies (ESSs) in the model. Instead, we find ESSs in which partial information is communicated, with discrete displays signalling a range of values rather than a precise one. The result also indicates that communication should be more precise in conflicts over small resources. Signalling strategies can exist in fighting because of the common interest in avoiding injuries, but communication is likely to be limited because of the fundamental conflict over the resource. Our results reflect a compromise between these two factors. Data allowing for a thorough test of the model are lacking; however, existing data seem consistent with the obtained theoretical results. Copyright 1998 The Association for the Study of Animal Behaviour.     

A survey of polyvinylphenol blend miscibility

The effect of polymer structure on blend miscibility with polyvinylphenol (PVPh) has been studied for at least one polymer from a variety of polymer classes with potential hydrogen-bond accepting groups: polyesters (aliphatic and aromatic), polycarbonates, polyimides, polyamides, polysulfones, polyurethanes, polyethers, polysiloxanes, poly(amideimides), and cellulose esters. Many of the polyesters, polyamides, and cellulose esters showed evidence of interaction and miscibility with PVPh. In most of the other cases, there was no sign of miscibility. Generally good correlation exists between thermal behavior and infrared spectral data. Where there is significant interaction seen between the polymers by FTIR, substantial miscibility is seen by DSC analysis. In some cases, the phase behavior depended on the blend preparation scheme. © 1994 John Wiley & Sons, Inc.     

Supermirror hard-x-ray telescope

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.