2. GaAs detectors for medical imaging G.I.Ayzenshtat a*, S.M. Gushin a, O.B. Koretskaya b, O.P. Tolbanov b, A.V. Tyazhev b, E.A. Babichev c, V.R. Groshev c, G.A. Savinov c, A.P. Vorobiev d a Research Inst. of Semicond. Dev., Tomsk, Russia b Siberian Physical Technical Institute, Tomsk, Russia c Budker Institute of Nuclear Physics, Novosibirsk, Russia d Institute of High Energy Physics, Protvino, Russia * ayzen@mail.tomsknet.ru

3. Low dose digital radiographic device with GaAs - detectors The aim of this work was to test GaAs detectors in a real radiographic device Gas detector was replaced by a GaAs one in a Low Dose Digital Radiographic Device “LDRD SIBERIA-N” Detector + Electronics Low dose digital radiographic device

4. The main properties of compensated SI-GaAs(Cr) detectors 1. SI-GaAs(Cr) detectors have no Schottky contacts, they have only ohmic ones. I-V characteristics are linear. 2.The electric field (E) occupies the whole volume of detector. E(kV/cm)  const 3. Charge collection in GaAs- detectors is determined by electrons. 4. The detector material resistivity is extremely high ( ρ>10 9 Ω∙cm). E  const

5. Attachment of GaAs-detectors to electronics I z = 0 I = +I 0 I = -I 0 +U 1 - U 1 z X-Rays The problem: the dark current of detector is about 10 nA. It is necessary to reduce the current by a factor of 10 for the electronics to operate. Solution of the problem: we offer to attach the detectors to electronics as shown in the figure.

6. Symmetrical detector structures with the compensation of dark currents The hypothetical detector structure The real structure of the mammo- graphy detector (100 mkm pitch) + U - U I = 0

7. The results of current compensation measurements in the symmetrical device. The output current of the detector decreases by a factor 30 X- rays screen +U 0 -U 0

8. Detector 1 Detector 2 Plane view of detector (h =  m) A fragment of hybrid circuit of electronics A fragment of detectors mounting 64-channel chip

9. The magnitude of the output signal as a function of the Roentgen tube current The magnitude of the output signal as a function of dosage rate for various quantum energy

10. Waveshape of the signal in vertical scanning of the detector SNR 2 as a function of dosage rate

11. «Channel form» obtained by vertical scanning UU Down To electronics X-rays

12. «Channel form» obtained by horizontal scanning Detector The collimator moves along the horizontal axis Collimator The normalized amounts of signals for various channels during horizontal scanning To electronics X-rays

13. Image of the test pattern obtained by the GaAs detector with a 400 mkm pitch (The limiting image resolution is 1.4 pairs of lines per mm)

14. Plane view of the detector (h = 200  m) fabricated from GaAs liquid-phase epitaxy Comparision of I-V characteristics for two detectors. 1. The GaAs liquid-phase epitaxy detector 2. The SI-GaAs(Cr) detector

15. IT The image of the test pattern obtained by GaAs detector with a 200  m pitch (The limiting image resolution is 2.8 pairs of lines per mm)

16. Conclusion 1. It was shown experimentally that the GaAs material compensated with Cr can be used to fabricate efficient X-ray detectors. 2.The object image with the resolution 2.8 pairs of lines per mm has been obtained with GaAs detectors. 3.We consider the GaAs(Cr) liquid-phase epitaxy material to be a more promising material for obtaining the resolution more than 2.8 pairs of lines per mm.