Tom Francke

Dose reduction in mammography with photon counting imaging

The purpose of this study was to investigate if the glandular dose to the breast in mammography can significantly be reduced without compromising image quality, when using photon counting technology, in a multi-slit scanning photon counting detector, compared to a conventional film mammography system and commercial available digital mammography systems with TFT-array detectors. A CDMAM phantom study, with two different thicknesses of additional PMMA absorber, 4 cm and 7 cm respectively, has shown that multi-slit scanning photon counting detector technology can reduce the dose, without reducing the image quality. This comparison was made to two commercial available digital mammography systems Senographe 2000D (from GEMS) and Selenia (from Lorad). The results show that dose can be reduced with 63% to 77%, depending on object thickness, when using XCT for mammography. This dose reduction has also been verified clinically through a small pilot study with patients and specimen, where the comparison was made between XCT and film.

Innovative applications and developments of micro-pattern gaseous detectors

I.

MICROPATTERN GASEOUS DETECTORS

Currently a revolution is taking place in the development of gaseous detectors of photons and particles. Parallel plate-type and wire-type detectors which dominated for years in high energy and space flight experiments are now being replaced by recently invented Micropattern gaseous detectors. We will now review the main achievements in this field and discuss the most promising directions in future developments and applications.

Yield of photoelectrons produced in TMAE and solid TMPD By BaF2 scintillation light

Abstract The yield of photoelectrons produced in gaseous TMAE and solid TMPD by BaF2 scintillation light and detected by multiwire proportional chambers operated at low pressure has been studied. The scintillation light was generated by collimated 511 keV gamma rays from a 22Na source. Using a new technique for analysing low-pressure MWPC spectra, it is shown that TMAE gives a spectrum corresponding to 9.6±0.5 photoelectrons while the quantum efficiency of solid TMPD is only 2% of that of TMAE. Adding TMP to lower the threshold of photoelectron emission, it is possible to double the efficiency of TMPD.

Compact fast rich detector using a solid radiator: Principle and first experimental results

Abstract The principle and first experimental results for a compact fast RICH detector using a solid radiator (NaF crystal) are presented. This detector can be very thin (≅ 4 cm) and presents a small percentage of radiation length. First experimental results for β = 0.82 and β = 1 particles at different incident angles indicate a good determination of the ring diameter. This type of particle identifier may be quite useful in compact medium energy experiments.

Conversion of UV and Visible Photons to Photoelectrons

The operation of most of gaseous photomultipliers is based either on gas photoionization or on photoelectric effect from solid photocathodes. There have also been attempts to use liquid photocathodes which offer lower ionization thresholds compared to the corresponding vapors. A great success has been achieved with solid photocathodes covered with adsorbed layers of some photosensitive vapors which reduce the cathode work function and as a result extend the photosensitivity threshold towards long wavelengths. It also enhances their quantum efficiencies sometime on a factor of two. The main physic mechanisms of interactions of UV photons with gases as well as with liquid and solid photocathodes are described in detail in this chapter. This basic knowledge is important when designing and using gaseous photodetectors.

Operation of Micropattern Gaseous Detectors

This chapter is dedicated to the physic of the operation of micropattern detectors. The authors analyze in more detail what causes discharges in these detectors. The chapter shows that, at low counting rates, the breakdowns appear due to the Raether limit and in some specific cases due to surface streamers. In some particular detectors (e.g. combined with high-efficient photocathodes or operating in very clean noble gases) the discharges may appear via a feedback mechanism. At high counting rates, the maximum achievable gain drops with the counting rate due to avalanches overlapping in space and time, and also due to a contribution from explosive electron emission. Detailed studies of the problems that micropattern detectors, in particular GEM, may experience while operating in cascade mode are presented. A better understanding of these effects has allowed researches to make a further step in the development of micropattern gaseous detectors in recent years.

Recent Developments of Micropattern Detectors

In this chapter, the exciting developments in micropattern detectors in recent years are described. This includes GEM and MICROMEGAS detectors combined with micropixel readout, some peculiar designs of GEM and GEM-like detectors sensitive to UV and visible light, large area (>1m2) GEM and MICROMEGAS prototypes developed for the upgrades of the experiments at the large hadron collider, etc. A special focus is put on a new generation of spark-proof micropattern detectors, using resistive electrodes instead of traditional metallic ones. These detectors operate as ordinary micropattern detectors. However, in the case of occasional sparks, their current is limited by the resistivity of the electrodes so that the energy of the discharge is reduced by several orders of magnitude. Various designs of such detectors have been developed and successfully tested, including resistive GEM, resistive MICROMEGAS, resistive MSGC, etc. Among this family of detectors, a special place belongs to resistive parallel-plate micropattern detectors allowing one to achieve at the same time excellent spatial (38 μm) and time (77 ps) resolutions. Finally, the potential of multilayer detector technology for further optimization of the detector operation is discussed.