Gravitational, magnetic and superfluid forces can stress the crust of an evolving neutron star. Fracture of the crust under these stresses could affect the star's spin evolution and generate high-energy emission. We study the growth of strain in the crust of a spinning down, magnetized neutron star and examine the initiation of crust cracking (a {\em starquake}). In preliminary work (Link, Franco & Epstein 1998), we studied a homogeneous model of a neutron star. Here we extend this work by considering a more realistic model of a solid, homogeneous crust afloat on a liquid core. In the limits of astrophysical interest, our new results qualitatively agree with those from the simpler model: the stellar crust fractures under shear stress at the rotational equator, matter moves to higher latitudes and the star's oblateness is reduced. Magnetic stresses favor faults directed toward the magnetic poles. Thus our previous conclusions concerning the star's spin response still hold; namely, asymmetric redistribution of matter excites damped precession which could ultimately lead to an increase in the spin-down torque. Starquakes associated with glitches could explain the permanent {\em offsets} in period derivative observed to follow glitches in at least three pulsars.Comment: 20 pages, 6 postscript figures, uses AASTeX macros, submitted to Ap

We report the discovery of nearly coherent oscillations with a frequency of ∼ 567 Hz during type-I X-ray bursts from the X-ray transient and eclipsing binary X 1658-298. If these oscillations are directly related to the neutron star rotation then the spin period of the neutron star in X 1658-298 is ∼1.8 ms. The oscillations can be present during the rise or decay phase of the bursts. Oscillations during the decay phase of the bursts show an increase in the frequency of ∼0.5 to 1 Hz. However, in one particular burst the oscillations reappear at the end of the decay phase at about 571.5 Hz. This represents an increase in oscillation frequency of about 5 Hz which is the largest frequency change seen so far in a burst oscillation. It is unclear if such a large change can be accommodated by present models used to explain the frequency evolution of the oscillations. The oscillations at 571.5 Hz are unusually soft compared to the oscillations found at 567 Hz. We also observed several bursts during which the oscillations are detected at much lower significance or not at all. Most of these bursts happen during periods of X-ray dipping behavior, suggesting that the X-ray dipping might decrease the amplitude of the oscillations (although several complications exist with this simple picture). We discuss our discovery in the framework of the neutron star spin interpretation.

Neutron production in radiotherapy facilities has been studied from the early days of modern linacs. Detailed studies are now possible using photoneutron capabilities of general-purpose Monte Carlo codes at energies of interest in medical physics. The present work studies the effects of modelling different accelerator head and room geometries on the neutron fluence and spectra predicted via Monte Carlo. The results from the simulation of a 15 MV Siemens PRIMUS linac show an 80% increase in the fluence scored at the isocentre when, besides modelling the components necessary for electron/photon simulations, other massive accelerator head components are included. Neutron fluence dependence on inner treatment room volume is analysed showing that thermal neutrons have a 'gaseous' behaviour and then a 1/V dependence. Neutron fluence maps for three energy ranges, fast (E > 0.1 MeV), epithermal (1 eV < E < 0.1 MeV) and thermal (E < 1 eV), are also presented and the influence of the head components on them is discussed.

We present a detailed study of the correlations between the burst properties and the inferred mass accretion rate for the X-ray transient MXB 1659[298. The bursts that exhibited oscillations were observed when the source was at a relatively high mass accretion rate, similar to what has been seen for other sources. However, owing to the limited number of observations at lower mass accretion rates, no bursts were observed at such accretion rates, and it is still possible that when MXB 1659[298 accretes at such low-mass accretion rates, bursts can occur that might still exhibit burst oscillations. No clear correlations were found between the di †erent burst properties and the accretion rate, in contrast to what has been found for KS 1731[260 and 4U 1728[34, but similar to what has been reported for Aql X-1. However, this lack of correlation for MXB 1659[298 and Aql X-1 might be due to the limited range of the mass accretion rate observed for those sources compared to KS 1731[260 and 4U 1728[34.