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Y.I Zakari, R.D. Mavunda, T.L Nam and R.J Keddy

VARIATIONS IN CVD DIAMOND DETECTOR’S RESPONSE TO RADIATIONS WITH THE CRYSTAL’S DEFECTS COMPARED WITH CALCULATED VALUES FROM MC code(PENELOPE) AT LOW ENERGY MAMMOGRAPHY X-RAY RANGE. Y.I Zakari, R.D. Mavunda, T.L Nam and R.J Keddy. YI Zakari* RD Mavunda, TL Nam and RJ keddy.

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Y.I Zakari, R.D. Mavunda, T.L Nam and R.J Keddy

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  1. VARIATIONS IN CVD DIAMOND DETECTOR’S RESPONSE TO RADIATIONS WITH THE CRYSTAL’S DEFECTS COMPARED WITH CALCULATED VALUES FROM MC code(PENELOPE) AT LOW ENERGY MAMMOGRAPHY X-RAY RANGE Y.I Zakari, R.D. Mavunda, T.L Nam and R.J Keddy

  2. YI Zakari* RD Mavunda, TL Nam and RJ keddy • DST/NRF Centre of Excellence in Strong Materials and School of Physics,University of the witwatersrand, Private Bag 3, PO wits 2050, Johannesburg, Republic of South Africa • *Post Doctoral fellow from Ahmadu Bello University, Zaria, Nigeria

  3. INTRODUCTION • CVD DIAMOND DETECTOR AS THE ‘STATE OF THE ART’ FOR FUTURE TECHNOLOGY • Motivation: Breast cancer reported to be the highest source of mortality in women (next to lung cancer) and that at present X-ray mammography screening may also induce cancer

  4. AIM • CHARACTERIZATION OF CVD DETECTORS • TO EVALUATE CVD DIAMOND RESPONSE TO ALPHA LOW ENERGY X-RAY • ALPHA SPECTROCOPY • MC CODE(PENELOPE) OF MAMMOGRAPHIC X-RAY RANGE

  5. INSTRUMENTS FOR CHARACTERISATION • BRUKER MICROWAVE BRIDGE ESP 380-1010 • VARIAN CARY 500 UV-Vis-NIR SPECTROMETER • JOBIN-YVON T64000 RAMAN SPECTROMETER • TOLEDO 654 TLD UNIT • KEITHLEY 237 FOR i-v CHARACTERISTICS

  6. CVD

  7. MATERIALS • SINGLE CRYSTAL CVD DIAMOND • POLYCRYSTALS: DETECTOR GRADE AND OPTICAL GRADE CVD DIAMOND • METALIZED CVD DIAMOND

  8. EXPERIMENTAL SETUP-1 • SPECIALLY CONSTRUCTED AMPLIFIER CUM HIGH VOLTAGE SYSTEM COUPLED TO PC • Am-241 ALPHA SOURCE +PRE-AMP. IN VACUUM. • ACQUISITION OF DATA USING APTEC SOFTWARE

  9. EXPERIMENTAL SETUP-2 • SENOGRAPHE 500T MAMMOGRAPHY X-RAY UNIT. • PTW DIADOS 11003-1121 REFERENCE DETECTOR • SAMPLE HOLDER WITH APPLIED FIELD ACROSS SAMPLE AND DIADOS DETECTOR • RECORDINGS FROM WELLHOFER DOSIMETRIE CU500 CONTROL COMPUTER 232C-A • DATA ACQUISITION SOFTWARE PACKAGE WP600 VERSION 4.26C

  10. RESULTS-1

  11. VARIATION OF TOTAL ALPHA COUNTS WITH UV(/cm) Absorption for detector and optical grade CVD

  12. Variation of total alpha counts with the single substitution nitrogen concentration (ESR) of optical grade CVD diamond

  13. Variation of total alpha counts with single substitution nitrogen concentration for detector grade CVD diamonds

  14. Variation of total alpha counts with the Raman broadening (FWHM) for optical grade CVD diamonds

  15. Variation of TL response with single substitutional nitrogen concentration (ESR) for detector and optical grade CVD diamonds

  16. Variation of TL emission with Raman broadening for optical grade CVD diamonds

  17. Variation of Alpha FWHM counts with averaged UV absorption values for both detector and optical grade and single crystal CVD diamonds

  18. Variation of FWHM of total count with averaged Raman broadening values for both detector and optical grade and single crystal CVD diamonds

  19. Variation of alpha FWHM with single substitutional nitrogen concentration (ESR) for detector and optical grade CVD diamonds

  20. Variation of experimental alpha count rate with energy for optical grade (DG) CVD diamonds

  21. A typical optical grade CVD diamond spectrum before (upper curve) and after (Lower curve) background subtraction

  22. Alpha spectrum from OG4 polycrystalline and single crystal (SC) CVD diamond samples showing energy peak.

  23. Alpha spectrum from detector grade DG3 and DG4 CVD diamonds showing the characteristic energy peak

  24. Summary1 of observations on Alpha interaction with defects in CVD • Consistent trend of alpha counts having +ve gradient with UV absorption and TL emission but a –ve gradient to the Raman broadening and and Ns. • Relatively high total alpha counts from OG CVD diamond may be associated with UV related defects and build-up effect. • Ns. in CVD diamond is seen to act as a recombination center due to the observed higher sensitivity (counting efficiency) with lower Ns.concentration

  25. Summary2 of observations on Alpha interaction with defects in CVD • For reasonable alpha spectroscopy, the values of nitrogen concentration, UV absorption and Raman broadening be as low as possible, but TL value must be highest. • In General for alpha spectroscopy the SC is the choice material or DG grade as substitute. • Otherwise for a detector with higher sensitivity and less expensive the OG CVD material could serve

  26. The alpha spectrum stripping methodology for a comparative evaluation of inherent spectrometric performance of CVD diamond • Level of defects in CVD diamond wafers affects their response to radiation( Nam et al 1991; Davies, 1994; Iakoubovski et al, 2002; Nebel, 2003 Mavunda ,2008) • Alpha interaction( primary and secondary) with the detector material to cause excitation and ionization of the electrons into e-hole pairs • Deceleration of the e-hole pairs produced in the field of alpha produces bremsstrahlung that interacts by Compton scattering to cause the observed fluctuations in the spectrum • The sensitivity of the optical grades results in the observed skewedness and deviation of the spectrum peak. • Electronic and statistical factors were also considered

  27. Formulation of stripping equation The specific ionization I(Ei) defined as /cm

  28. Variation in peak efficiency with energy

  29. Variation in alpha count rate s with energy for an Optical grade CVD diamond (OG1)

  30. Variation in alpha count rate with energy for an optical grade CVD diamond (OG2)

  31. Variation of Alpha count rate with energy for a detector grade CVD diamond (DG1)

  32. Variation in Alpha count rate with energy for a particular detector grade CVd diamond (DG3)

  33. Variation in Alpha count rate with energy for a single crystal CVD diamond

  34. Table 1: Spectral Analysis of accumulated alpha spectra from the three grades of CVD diamonds (The energy of the impinging alpha particles being 5.47MeV)

  35. Summary of spectrometric analysis • Stripping method used was observed to have improved the detector peak resolution but without any effect on the peak efficiency of the detector. • The technique has more effect on the OG CVD diamond detectors than the DG and SC CVD detectors. The observed absolute efficiency above 100% due to build-up effect and fluctuation were stripped off to have a more realistic value • At the peak energy of 5.48 MeV a range of 90μm was calculated indicating the interaction is on the surface. • A mass stopping power of 0.281 MeV/mgcm-2 and a leakage current of betwn 5.2pA and 54.2 pA observed could classify these detectors as semiconducting • An average full energy peak efficiency of 80% both experimentally and analytically indicates that the stripping formular validate and improves the experimental data without any effect on the detectors inherent characteristic performance

  36. CVD detector’s responses with crystal defects at low energy mammographic X-ray range • Aim: To determine the effects of impurities in diamond on its performance as a mammographic X-ray detector. • To assay the I-V characteristics • Use the MC code to model the experimental set up carbon in place of diamond

  37. Current-voltage characteristics for DG, OG and SC diamond detectors

  38. Characterization results of CVD diamod as earlier reported Mavunda et al,2008)

  39. X-ray response rate at 200V bias for DG, OG and SC diamonds

  40. X-ray response rate at 300V bias for DG, OG and SC diamonds

  41. X-ray response rate at 400V bias for DG, OG and SC diamonds

  42. Averaged CVD diamond response to X-ray in calculated dose (Gy) values

  43. Variation of current with bias voltage for DG, OG and SC diamonds

  44. Variation of sensitivity with bias voltage for DG, OG and SC diamonds

  45. Variation of sensitivity with UV absorption for DG and SC CVD diamond at 200V bias

  46. Variation of sensitivity with single substitutional nitrogen concentration for DG CVD diamonds at 200V bias

  47. Variation of X-ray response rate with TL emission for DG and OG CVD diamond detectors

  48. variation of X-ray response rate with Raman broadening for DG and OG CVD diamond detectors

  49. Variation of X-ray response rate with single substitution nitrogen concentration for DG and OG CVD diamond detectors

  50. Variation of X-ray response rate with UV absorption for OG CVD diamond detectorsat voltage peak of 22 kVp and 200V bias

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