J. Ostling

Study of hole-type gas multiplication structures for portal imaging and other high counting rate applications

We performed studies in the operation of various hole-type gaseous multiplication structures (GEM, capillary plates, and others) at very high fluxes of X-rays (<60 keV-bremsstrahlung) and pulsed gamma radiation (<50 MeV-bremsstrahlung). In the case of X-rays, the counting rate was 10/sup 5/-10/sup 6/ Hz/mm/sup 2/; in the case of gammas it reached levels of up to 10/sup 9/ Hz/mm/sup 2/ during the pulses. In most measurements, the entire area of the detector was exposed to these extremely high fluxes. Results obtained so far indicate that the GEM and the hydrogen-treated capillary plates could be successfully used in medical imaging applications.

The study and optimization of new micropattern gaseous detectors for high rate applications

We performed a new series of systematic studies of gain and rate characteristics of several micropattern gaseous detectors. In contrast to earlier studies, these measurements were done at various pressures, gas mixtures, at a wide range of primary ionization and also when the whole area of the detectors was irradiated with a high intensity X-ray beam. Several new effects were discovered common to all tested detectors, and which define fundamental limits of their operation. Results of these studies allow us to suggest some concrete ways of micropattern detectors improving.

A novel portal imaging device for advanced radiation therapy

At Karolinska Institutet in Stockholm, Sweden a new detector for portal imaging is under development, which could greatly improve the alignment of the radiation beam with respect to the tumor during radiation treatment. The detector is based on gaseous and solid converters combined with GEMs as an amplification structures. The detector will have a large area and will be operated in a very high rate environment. A prototype has been built and extensively tested, both with high rate X-rays as well as in a clinical environment at the Karolinska Hospital. High rates and alpha particles could cause discharges in the GEM and discharge propagation from GEM to GEM and to the readout electronics. Since reliability is one of the main requirements for the portal imaging device, the authors performed systematic studies to find a safe operating range of the device, free from typical high-rate problems, such as discharges.

Evaluation of a GEM and CAT-based detector for radiation therapy beam monitoring

We are developing a radiation therapy beam monitor for the Karolinska Institute. This monitor will consist of two consecutive detectors confined in one gas chamber: a keV-photon detector, which wil ...

Novel detector for portal imaging in radiation therapy

We are developing a novel concept for portal imaging that would allow for on-line control and verification of the radiation treatment of cancer patients both at diagnostic and therapeutic energies. This device will consist of two consecutive detectors confided in one gas chamber: a KeV- photon detector, which can visualize the internal soft tissue of the patient, and an MeV-photon detector, which will measure the absolute intensity of the therapeutic beam and its position with respect to the tumor and normal tissues. Both detectors are based on gas and solid photon to electron converters combined with recently invented gas electron multipliers. The device will have a common charge collecting pad-type readout plate equipped with ASIC-based electronics for both detectors. A first simplified prototype device has recently been built and extensively tested. Special efforts were made to find conditions for a safe and reliable operation of the readout electronics that can be damaged by plasma-type discharge effects induced specially at high dose rates. Results obtained so far indicate that our new detector concept may satisfy all requirements on advanced therapy beam monitoring systems.

Operating range of a gas electron multiplier for portal imaging

At the Karolinska Institute in Stockholm, Sweden a new detector for portal imaging is under development, which could greatly improve the alignment of the radiation beam with respect to the tumor during radiation treatment. The detector is based on solid converters combined with gas electron multipliers (GEMs) as an amplification structure. The detector has a large area and will be operated in a very high rate environment in the presence of heavy ionizing particles. As was discovered recently high rates and alpha particles could cause discharges in GEM and discharge propagation from GEM to GEM and to the readout electronics. Since reliability is one of the main requirements for the portal imaging device, we performed systematic studies to find a safe operating range of the device, free from typical high rate problems, such as discharges.

Study of capillary-based gaseous detectors

We have studied gain vs. voltage characteristics and position resolutions of multistep capillary plates (two or three capillary plates operating in a cascade), as well as capillary plates operating in a mode where the main amplification occurs in between the capillary plate and the readout plate (parallel plate amplification mode). Results of these studies demonstrated that in parallel-plate amplification mode one can reach both high gains (>10/sup 5/) and good position resolutions (/spl sim/100/spl mu/m) even with a single step arrangement. This offers a compact amplification structure which can be used in many applications.

A high position resolution X-ray detector: an "Edge on" illuminated capillary plate combined with a gas amplification structure

We have developed and successfully tested a prototype of a new high position resolution hybrid X-ray detector. It contains a thin-wall lead glass capillary plate converter of X-rays combined with a microgap parallel-plate avalanche chamber operating in various gas mixtures at 1 atm. The operation of these converters was studied in a wide range of X-ray energies (from 6 to 60 keV) at incident angles varying from 0-90/spl deg/. The detection efficiency, depending on the geometry, photons energy, incident angle and the mode of operation was between 5-30% in single step mode and up to 50% in a multi-layered combination. Depending on the capillarys geometry, the position resolution achieved was between 50-250 /spl mu/m in digital form and was practically independent of the photons energy or gas mixture. The usual lead glass capillary plates operated without noticeable charging up effects at counting rates of 50 Hz/mm/sup 2/ and hydrogen treated capillaries-up to 10/sup 5/ Hz/mm/sup 2/. The developed detector has several important potential advantages over the exciting X-ray detectors and may open new possibilities for medical imaging, for example in mammography, portal imaging, radiography (including security devices), as well as many other applications.