Mittuniversitetet

Radon gas, Rn-222, is a problem in many buildings. The radon gas is not harmful in itself, but the decay chain contains charged elements such as Po-218, and Po-214 ions which have a tendency to stick to the lungs when inhaled. Alpha particles from the decay of these ions cause damages to the lungs and increase the risk of lung cancer. The recent reduction in the limits for radon levels in buildings call for new simple and efficient measurement tools [1]. The system has been optimized through modifications of the detector size, changes to the filters and the design of the chamber. These changes increase the electric field in the chamber and the detection efficiency.

Pixellated silicon detectors with a pixel size of 220 um have been fabricated at Mid Sweden University. The detectors will be bonded to the MEDIPIX2 [1] readout chip. The purpose is to investigate the performance of an energy sensitive X-ray imaging sensor with reduced charge sharing.

The detectors were fabricated on high purity silicon with a wafer thickness of 500 um and a resistivity of more than 15 kohmcm. One reason for the choice of material was to get experience for future work with very thick detectors requiring ultra high resistivity in order to be depleted. During the initial work in this project some issued were found concerning inter pixel resistance and the efficiency of the guard rings. This led to a study of existing papers on the subject [2,3,4,5] and to extensive simulations of the electric field and the charge transport in different parts of the device.

A modified process has been developed using alternating p+ and n+ guard rings and an outer n+ doping. The results of the simulations and the process will be described as well as an outline for a process for fabrication of very thick detectors with limited guard ring extension.

References[1] - X. Llopart, M. Campbell, R. Dinapoli, D. San Segundo and E. Pernigotti, IEEE Trans. Nucl. Sci., vol. 49, 2279-2283, October 2002.[2] – L. Evensen, A. Hanneborg, B Sundby Avset, M. Mese, Nuclear Instruments and Methods in Physics Research A 337 (1993) 44 – 52[3] – T. Pavalainen, T. Tuuva, K. Leinonen, Nuclear Instruments and Methods in Physics Research A 573 (2007) 277 – 279[4] – Z. Li, W. Huang, L. J. Zhao, IEEE Trans. Nucl. Sci., vol. 47, No. 3. 729 – 735 , June 2000.[5] – D. Han, C. Wang, G. Wang, S. Du, L. Shen, X. Tian, X. Zhang, IEEE Transactions on Electron devices, Vol. 50, No. 2, 537 – 540, February 2003

The standard detector used with the MEDIPIX2 [1] readout chip suffers from severe charge sharing that reduces the spectral performance of the device. This problem is expected to be solved in the MEDIPIX3 [2] design. In order to significantly reduce the charge sharing and to make a detector which could be expected to have a similar response to MEDIPIX3 we have fabricated detectors with a pixel size of 220 um and bonded these detectors to the MEDIPIX2 chip using only a limited number of pixels on the readout chip. This makes the active area of the pixel comparable with the area covered by the charge summing in MEDIPIX3.

The charge collection properties of the device have been tested by scanning a narrow beam over a pixel. The spectral response has been measured by taking a flood exposure at different tube voltages and comparing the result with the spectrum obtained from exposing the centre of the pixel with a narrow beam thus eliminating the charge sharing. This work represents an improved characterisation as compared to [3].

Some initial images of different objects have been taken by placing the device in an X-ray microscope with a nanofocus X-ray tube imaging objects with magnification to simulate the original pixel size of 55 um.

References[1] - X. Llopart, M. Campbell, R. Dinapoli, D. San Segundo and E. Pernigotti, IEEE Trans. Nucl. Sci., vol. 49, 2279-2283, October 2002.[2] - R. Ballabriga, M. Campbell, E. H. M. Heijne, X. Llopart, L. Tlustos, IEEE Trans. Nucl. Sci., Vol. 54, No. 5, October 2007.[3] – B. Norlin, C. Fröjdh, G. Thungström, D. Greiffenberg, NSS Conference record,19-25 Oct. 2008, Pages:3464 – 3469, Digital Object Identifier 10.1109/NSSMIC.2008.4775083

An alpha particle detector for measuring radon levels through measurement of radon progeny concentration has been developed. The detector is a silicon diode optimized for these measurements. Different alternating and non-alternating guard ring structures and different doping profiles have been investigated.

Color informatics to analyze the object content in Xray images is an emerging technology. Identification of different elements for applications such as medical contrast agent imaging is possible using energy resolving X-ray imaging sensors. In this work RGB representations of transmission images of ground elements achieved with the MEDIPIX system are exemplified.

Energy resolved X-ray imaging can be used as a tool to analyze the variation in the chemical content of an object. In this work we have used energy resolved X-ray imaging to measure the variation in the chemical content of paper and paper coating. This is an important quality parameter for the paper industry. In order to separate the variation in coating thickness from the variation in paper thickness, energy resolution is used to separate the response of the coating from the response of the paper. The MEDIPIX2 single photon processing X-ray imaging system [1] has been used in the measurements.  The measurement results are compared to simulations with MCNP. The influence of charge sharing is discussed and the effects have been studied by comparing results from detectors with 220x220 µm² pixels and detectors with 55x55 µm² pixels. There is a trade-off between good spatial resolution obtained with detectors with small pixels and good energy resolution obtained with detectors with large pixels. The requirements on image quality, to achieve the resolution of coating distribution relevant for the application, are discussed.

X-ray imaging detectors with energy resolution, based on a single photon processing CMOS readout circuit attached to a detector chip, are being developed by different groups. In order to achieve high quantum efficiency it is preferable to use high-Z semiconductor materials. However the fluorescent X-ray photons of such materials have high energies and are able to travel long distances thereby reducing both the spatial resolution and the energy resolution of the detector. In addition charge sharing in the detector and non-uniformities in both the detector and the readout electronics will affect the signal. In this work we have characterized a 1 mm thick CdTe detector with a pixel size of 55 um x 55 um, bump bonded to the MEDIPIX2 single photon processing readout chip. The area sensitivity of the detector is evaluated using a narrow X-ray beam of monoenergetic photons. From these measurements the effects of fluorescence and charge sharing can be evaluated.