A miniature, battery operated 40 kV x-ray device has been developed for the interstitial treatment of small tumors ( < 3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe that is stereotactically inserted into the tumor. The beam, characterized by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 microA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

A device that generates low-energy x rays at the tip of a needle-like probe was developed for stereotactic interstitial radiosurgery. Electrons from a small thermionic gun are accelerated to a final energy of up to 40 keV and directed along a 3 mm outside diameter drift tube to a thin Au target, where the beam size is approximately 0.3 mm. All high-voltage electronics are in the probe housing, connected by low-voltage cable to a battery-operated control box. X-ray output, which is nearly isotropic, consists of a bremsstrahlung spectrum and several lines between 7 and 14 keV, with characteristic radiation contributing 15% of the total energy output. To date, 14 patients with metastatic brain tumors have been treated with this device.

In a polycrystalline ferromagnetic film, the magnetization direction is not uniform but exhibits small wave-like fluctuations known as magnetization ripple. In this paper, a general theory of ripple is developed, extending previous treatments by the present author, Hoffmann, and others. The theory is applicable to almost any magnetic film without gross domain structure, containing randomly oriented local anisotropies arising from inhomogeneities on any scale and of any physical origin. The magnetization direction is assumed to fluctuate in the plane of the film only. In addition to local anisotropy fields, the theory includes the effects of magnetostatic and exchange interactions, and a uniform field that consists of uniform uniaxial anisotropy and external fields. Nonlinear magnetostatic and uniform fields, which in previous treatments have been either neglected or treated as small perturbations, are taken fully into account through third-order torque terms. The ripple spectrum is derived, and from it a physical picture of the ripple is obtained in terms of a coupled region which (except for very large-scale inhomogeneities) is an elongated rhombus with long axis perpendicular to the mean magnetization. The spectrum contains a nonlinear effective field, which is evaluated in several limiting cases determined by the scale of inhomogeneity, the film thickness, an exchange length, and a magnetostatic length. The magnetization dispersion (mean ripple amplitude) is also obtained for these limiting cases, but no attempt is made to calculate other measurable magnetic-film properties. The results show: a weakening of first exchange and then magnetostatic forces as the scale of inhomogeneity increases, with the consequence that the dispersion shows an increasing dependence on the uniform field; a weakened dependence of the dispersion on both the magnitude of the local anisotropy and the uniform field for large-amplitude, nonlinear ripple; and a thickness-independent ripple in the thick-film limit, where the magnetostatic field is given by its bulk value.

A miniature, interstitial x-ray generator has recently been developed and is currently undergoing clinical trials for the treatment of brain tumors. The maximum photon energy from this x-ray tube is 50 keV, although most of the initial testing has been carried out at 40 keV. Dose rates of up to 2 Gy/min in a water phantom at a distance of 10 mm from the tube tip are produced. In this paper we describe the modeling and simulation of x-ray production from this device using the ITS 3.0 Monte Carlo code. Verification of the simulation of x-ray production in the device was carried out by comparing predictions of spatial photon distribution, energy spectrum, and dose versus depth in water with experimentally obtained measurements. Agreement between the simulated results and experimental measurements was fairly good when comparing the angular distribution of photons emitted from the x-ray tube and very good when comparing dose rate versus depth in a water phantom. Discrepancies observed when comparing the calculated and measured estimates of characteristic line radiation were reduced by incorporation of a modification to the ITS code. Possible causes of the remaining discrepancy in bremsstrahlung intensity are discussed.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 130.70.241.163 On: Tue, 23 Dec 2014 01:48:37 PLASMA POTENTIAL MEASUREMENTS 1399 ACKNOWLEDGMENTSThese experiments owe their origin to a suggestion by R. A. Dandl. The members of the DCX-l group who are responsible for the guccessful operation of the facility, which in tum is responsible for the completion of these experiments, are

The nonlinear reaction of dispersion-induced spin waves on the uniform mode m0 has been calculated for a thin film undergoing rapid rotational magnetization reversal. It is found that if m0 rotates faster than longitudinal spin waves can relax, the magnetization goes through a transient state of high magnetostatic energy. If the pulse switching field Hp is less than a critical field Hpc, the uniform mode becomes locked at some point in the reversal process; rotational switching cannot proceed until initially longitudinal spin waves have relaxed into components propagating in the instantaneous direction of m0, a highly damped process suggestive of the intermediate-speed reversal mode observed in thin films. The dependence of Hpc on a dc bias field has been verified experimentally; the dispersion dependence should provide a crucial test of the theory.

A theory of planar fluctuations of the direction of magnetization in a thin film which takes into consideration the nonlinear longitudinal magnetostatic force is outlined, and the resultant ripple is compared with that predicted by linear theories. The longitudinal magnetostatic force arises from the change along the direction of mean magnetization m0 of the component of magnetization M(r) parallel to m0; it leads to a torque that varies as the cube of the ripple amplitude. As a result of this torque, the linear theory breaks down for dispersion δ≳δ1, where δ is the rms angular deviation of M from m0 and δ1 is typically only 1° or 2° for a 1000 Å Permalloy film in zero field and varies as L−½, where 2L is the film thickness. With the inclusion of this force the range of validity of the theory has been extended to δ≲20°. A number of interesting effects have been found, including (1) an effective field in the direction of m0 (and proportional to δ4 for δ&gt;δ1) acting on ripple components with wavelengths greater than 4πL; (2) an effective field 4πm0δ2, acting on shorter wavelength components; (3) as a result of (1), a decrease in the longitudinal and transverse ripple coherence lengths rl and rt (or, depending on viewpoint, a decrease in the longitudinal and transverse cutoff wavelengths); (4) a dispersion δ∝ (Krc)2/5, where K=local anisotropy energy and rc=crystallite size (in the linear theory δ∝ Krc); (5) almost no change in the mean ripple wall spacing as observed by Lorentz microscopy, ∼πrc, provided rc&lt;rl; and (6) a spin-wave reaction torque coefficient in high-speed switching R∝δ10/3 (in the linear theory R∝δ2).

The relaxation time τ for realignment of an atomic anisotropy by 90° has been measured for a number of fast processes (τ&lt;103 sec) occurring in nonmagnetostrictive Permalloy films. The method of measurement utilizes the magnetoresistance effect and is described in detail elsewhere. For films deposited between 23° and 200°C at 10−6 to 10−5 Torr and measured at the same temperature and pressure, three distinct processes plus two probable ones have been found, which together account for about one-quarter of the total anisotropy. The activation energy Q and period factor τ0 which determine the relaxation time τ=τ0 exp (Q/kT) have been determined for each of these processes.