Bernhard Ludewigt

Designing accelerator-based epithermal neutron beams for boron neutron capture therapy

The 7Li(p,n)7Be reaction has been investigated as an accelerator-driven neutron source for proton energies between 2.1 and 2.6 MeV. Epithermal neutron beams shaped by three moderator materials, Al/AlF3, 7LiF, and D2O, have been analyzed and their usefulness for boron neutron capture therapy (BNCT) treatments evaluated. Radiation transport through the moderator assembly has been simulated with the Monte Carlo N-particle code (MCNP). Fluence and dose distributions in a head phantom were calculated using BNCT treatment planning software. Depth-dose distributions and treatment times were studied as a function of proton beam energy and moderator thickness. It was found that an accelerator-based neutron source with Al/AlF3 or 7LiF as moderator material can produce depth-dose distributions superior to those calculated for a previously published neutron beam design for the Brookhaven Medical Research Reactor, achieving up to approximately 50% higher doses near the midline of the brain. For a single beam treatment, a proton beam current of 20 mA, and a 7LiF moderator, the treatment time was estimated to be about 40 min. The tumor dose deposited at a depth of 8 cm was calculated to be about 21 Gy-Eq.

A high rate, low noise, X-ray silicon strip detector system

An X-ray detector system, based on a silicon strip detector wire-bonded to a low noise charge-sensitive amplifier integrated circuit, has been developed for synchrotron radiation experiments which require very high count rates and good energy resolution. Noise measurements and X-ray spectra were taken using a 6 mm long, 55 /spl mu/m pitch strip detector in conjunction with a prototype 16-channel charge-sensitive preamplifier, both fabricated using standard 1.2 /spl mu/m CMOS technology. The detector system currently achieves an energy resolution of 350 eV FWHM at 5.9 keV, 2 /spl mu/s peaking time, when cooled to -5/spl deg/C. >

Silicon Detectors for Low Energy Particle Detection

Silicon detectors with very thin entrance contacts have been fabricated for use in the IMPACT SupraThermal Electron (STE) instrument on the STEREO mission and for the Solid State Telescopes on the THEMIS mission. The silicon diode detectors were fabricated using a 200 Aring thick phosphorous doped polysilicon layer that formed the thin entrance window. A 200 Aring thick aluminum layer was deposited on top of the polysilicon in order to reduce their response to stray light. Energy loss in the entrance contact was about 350 eV for electrons and about 2.3 keV for protons. The highest detector yield was obtained using a process in which the thick polysilicon gettering layer was removed by chemical etching rather than chemical mechanical polishing.

Observation of beta and X rays with 3-D-architecture silicon microstrip sensors

The first silicon radiation sensors based on the three-dimensional (3-D) architecture have been successfully fabricated. X-ray spectra from iron-55 and americium-241 have been recorded by reading out a 3-D architecture detector via wire bonds to a low-noise, charge-sensitive preamplifier. Using a beta source, coincidences between a 3-D sensor and a plastic scintillator were observed. This is the first observation of ionizing radiation using a silicon sensor based on the 3-D architecture. Details of the apparatus and measurements are described.

An 8/spl times/8 pixel IC for X-ray spectroscopy

An integrated circuit (IC) providing 64 channels of low-noise signal processing electronics in an 8/spl times/8 pixel arrangement has been developed as part of an integrated silicon detector array for high count-rate X-ray spectroscopy applications. Each pixel features low-noise charge integration, programmable peaking time and gain, and an output driver. The 8/spl times/8 pixel IC builds upon our previous development of the XPS chip, a 1-dimensional (1-D) preamplifier-shaper IC for linear silicon detector arrays. The new pixel design features significant improvements to the shaper and output driver stages, including digital peaking time and gain selection, and a low-power charge driver/receiver design. When operated with a cooled, low-capacitance silicon detector, an energy resolution of /spl sim/210 eV FWHM was obtained for 5.89 keV X-rays. The power dissipation is /spl sim/6 mW per pixel at a supply voltage of 3.3 V.

Spectral response of multi-element silicon X-ray detectors

An X-ray spectrometer combining multi-element silicon detectors and multi-channel integrated circuit pulse-processing electronics is being developed for low noise, high count rate synchrotron X-ray fluorescence applications. This paper reports on the issues surrounding the use of highly segmented silicon detectors for X-ray spectroscopy. Several different detector geometries were modeled using commercially available device simulation software, and selected geometries were fabricated using planar processing techniques on high resistivity silicon. The detectors were characterized using a 5 /spl mu/m diameter 8.5 keV X-ray beam, and /sup 55/Fe and /sup 109/Cd radioisotope sources. Spectral background, anomalous peaks, peak-to-background and charge sharing between adjacent detector elements were studied. The measured X-ray spectral responses are interpreted with respect to the device simulations. These measurements bring to light the effects of detector design, detector processing techniques and detector materials properties on the spectral response of the detector.

Preliminary results of a raster scanning beam delivery system

A beam delivery system using a raster scanning technique has been developed to create large radiation fields with light ion beams for radiation treatment of cancer patients. Radiation fields up to 20 cm by 30 cm have been produced with a beam of Ne ions having a magnetic rigidity of 6.9 T-m. The radiation fields produced by this method appear to be less sensitive to beam position and beam shape than the wobbler dynamic beam delivery system, but require more stringent beam spill requirements to insure uniform radiation fields.<<ETX>>

A next generation, high speed detector for synchrotron radiation research

A high-speed, one-dimensional detector array for electrons and UV/X-ray photons has been developed. The detector is capable of handling the high countrates encountered in at third generation synchrotron radiation sources and is free from nonlinearity problems present in charge coupled device (CCD) detectors. Electrons are counted by a configuration of microchannel plates, an array of charge collection electrodes, and custom-designed integrated circuits (IC) assembled on a ceramic hybrid. The charges are collected on 768 strips with a 48 /spl mu/m pitch that are wire-bonded to 6 pairs of signal processing ICs. Each front-end IC has 128 channels of amplifiers (peaking time 25 ns) and discriminators. The pulse-pair resolution is 50 ns leading to a maximum linear countrate/channel of 2 MHz. The second, custom-designed IC features 24-b buffered counters and a serial link for the transfer of commands and data. A possible deadtime-less readout of all channels in 150 /spl mu/s opens the door to time resolved experiments. The complete detector system includes the high-voltage power supply, a field programmable gate array (FPGA)-based data acquisition system, and supporting software. Special care has been taken to insure reliable operation in an ultra-high vacuum environment. The detector architecture and design is described and measured performance characteristics such as spatial resolution and count-rate linearity are presented.

Measurements of beam current density and proton fraction of a permanent-magnet microwave ion source

A permanent-magnet microwave ion source has been built for use in a high-yield, compact neutron generator. The source has been designed to produce up to 100 mA of deuterium and tritium ions. The electron-cyclotron resonance condition is met at a microwave frequency of 2.45 GHz and a magnetic field strength of 87.5 mT. The source operates at a low hydrogen gas pressure of about 0.15 Pa. Hydrogen beams with a current density of 40 mA/cm(2) have been extracted at a microwave power of 450 W. The dependence of the extracted proton beam fraction on wall materials and operating parameters was measured and found to vary from 45% for steel to 95% for boron nitride as a wall liner material.

A sixteen channel peak sensing ADC for singles spectra in the FERA format

To read out multi-element small X-ray detectors for X-ray fluorescence applications with synchrotron radiation one needs the capability to record multiple singles spectra for each detector element at high rates. We have developed a sixteen channel 11 bit peak sensing ADC in a CAMAC module. We use the FERA readout bus to place the data into a commercially available histogramming module developed to generate multiple histograms from FERA ADCs. The sixteen channels digitize shaped pulses from the detectors without external gating. The digitizing time is 8 /spl mu/sec, the peak acquisition time is /spl ges/2 /spl mu/sec. The module contains a LIFO to permit block transfers in order to minimize dead times associated with the readout. There is a common CAMAC controlled analog threshold for noise suppression and a 16 bit mask to enable or disable individual ADCs. Differential non linearity is less than +8%/-4%. A /spl gamma/-ray spectrum collected using this ADC is presented.