Backscattering of Electrons from 3.2 to 14 MeV: Reflection of Experimental Method and Errors

1. EGS Study Meeting at KEK, August 8, 2007 Backscattering of Electrons from 3.2 to 14 MeV: Reflection of Experimental Method and Errors Tatsuo Tabata Osaka Prefecture University and Institute for Data Evaluation and Analysis

2. Introduction • T. Tabata, “Backscattering of Electrons from 3.2 to 14 MeV.” Phys. Rev. 162, 336—347 (1967) • Published just 40 years ago! • To be used as a benchmark for Monte Carlo calculations, description is given of: • Experimental method • Evaluation of errors

3. Introduction (continued) • As another benchmark, a brief mention is given of: • Our experimental data on depth profiles of charge deposition

4. Backscattering: Experimental Method • Electron beam • Scattering chamber • Target and target assembly • Ionization chamber and measurements • Background • Secondary electrons

5. Electron beam • Electron beam produced by: • Linac of the former Radiation Center of Osaka Prefecture

6. Electron beam (continued) • Analizing magnet:Calibrated within an error of 1.1% • Quadrupole magnets:Focused the beam on the entrance collimater of the scattering chamber 5.5 m away

7. Electron beam (continued) • Collimator: copper, 160 mm in length • Beam at the target: • Energy spread, about 1% • Angular divergence, less than 0.05º • Diameter, 6.5 mm • Energy calibration: • Conversion electron line of Cs-137 • Cu-63 (g, n) reaction

8. Scattering chamber • Fixed lid and rotatable cylindrical box, each 50 cm i.d., 15 cm high, made of stainless steel • Measuring port, attached to the box with a dip of 20º from the horizontal plane

9. Scattering chamber (continued) • The rotation of the box: • Under vacuum • With remote control of a drive motor • The angular position 0 of the measuring port • Known to 0.2º at the control panel • Scattering angle, given bycosq= cos20ºcosq0

10. Scattering chamber (continued) • Vacuum of the chamber • Order of 10–3 Pa • Target • Hung with a rod from the center of the lid • Insurated to measure the target current • Measuring port • Mylar window, 3.5-mg/cm2 thickness

11. Scattering chamber (continued) • Detector collimator • Made of copper • With a conical taper matching the solid-angle cone subtended at the center of the target surface • Solid angle of detection: 1.92x10–4 sr

12. Targets and target assembly • Targets • Purity better than 99.5% • Mounted on a ring-shaped copper holder and a ceramic insulator • Placed perpendicular to the beam,the center of the incident surface being at the center of the scattering chamber (SC)

13. Targets and target Assembly (continued) • Thin targets • Backed with an aluminum Faraday cup,having an entrance hole of 11 mm in diameter and 35 mm in depth

14. Ionization chamber and measurements • Ionization chamber (IC) • X-ray compensation type developed by Van de Graaff et al., Phys. Rev. 69, 452 (1946) • Structure and operation of IC: • Charge collector; aluminum plate 60 mm in diameter, 30 mm thick, sandwiched between two aluminum foils 27 mg/cm2 thick, with gaps of 4 mm • High voltages of opposite polarities applied to the foils reduced X-ray BG.

15. Ionization chamber and measurements (continued) • Remotely controlled shutter in front of IC • For measuring uncompensated portion of BG. • Made of copper plate of 40 mm in diameter and 10 mm thick

16. Ionization chamber and measurements (continued) • Block diagram of measurements

17. Ionization chamber and measurements (continued) • Multiplication factorf of the IC • Assumed to depend only on the average energy Eav(E0, Z) per backscattered electron • Eav(E0, Z) was estimated from Wright and Trump (1962) by logarithmic extrapolation.

18. Ionization chamber and measurements (continued) • Calibration of f • From the ratio of fIb measured with the IC to Ib measured by a Faraday chamber (FC) for a thick Au target • FC, consisting of an Al collector of 60 mm in diameter and 30 mm thick, being directly attached to the measuring port of SC.

19. Ionization chamber and measurements (continued) • Calibration of f (continued) • Correction for FC efficiency, for the backscattering and secondary emission:4.1–8.9% depending on incident energy E0

20. Ionization chamber and measurements (continued) • The calibration curve obtained

21. Background • BG uncompensated in IC • Measured by closing the shutter • Smaller BG of another type, SEs produced near the measuring port of SC by bremsstrahlung from the entrance collimator • Studied for each E0 without the target • The total BG: always highest at 160º; 0.5–20% of the signal.

22. Background (continued) • BG for the FC used for calibration • Measured by inserting an Al plug35 mm long in the detector collimator • 2–12% at 160º, increasing with increasing E0

23. Secondary electrons • Secondary emission coefficient d: • Necessary for the correction of the target current It • Measured with the aid of a ring-shaped electrode attached to the incident side of the target

24. Backscattering: Evaluation of Errors • Possible sources of systematic errors and their values • Six items • Total error in backscattering coefficients • Dressel’s gross errors • Comparison with compiled experimental data and ITS Monte Carlo results

25. Possible sources of systematic errors and their values • FC efficiency, including the assumption that fwas a function of Eav only: ±2.9–8.1% error in f depending on E0 and Z • Solid angle of detection: ±1.8% error in  • Possible change of d due to electron bombardment:±10% error in d • Unmeasured fraction of BG:±1% error in Ii(q)

26. Possible sources of systematic errors and their values (continued) • Secondary emission from the target caused by bremsstrahlung, and re-backscattering of electrons from the walls of SCto the target:±0.5% error in It • Relative indication of thecurrent integrator and the picoammeter: ±1.5% error in Ii(q)/It

27. Total error in backscattering coefficients • As shown in Tables I and II of the paper,6.7–14% depending on E0 and Z • No problem has been found in the evaluation of errors by the present review.

28. Dressel’s gross errors • R. W. Dressel. “Retrofugal electron flux ...” Phys Rev. 144, 332 (1966) • Backscattering coefficients about 2 times of earlier authors’ and our results. • The cause of those gross errors was later found by himself to be the halo of the beam,which was incident on the target but missed by the Faraday cup to calibrate the beam monitor.

29. Dressel’s gross errors (continued) • Dressel’s experimental arrangement • Beam monitor: located in front of a collimator • Collimator: produced the peripheral halo of the beam

30. Dressel’s gross errors (continued) • Beam profile and the peripheral hallo of the beam Figure, Dressel’s private communication

31. Comparison with compiled experimental data and ITS Monte Carlo results • Figures: R. Ito et al., Phys. Chem. 42, 761 (1993) • Numerical ITS results and an empirical formula: R. Ito et al., Bull. Univ. Osaka Pref. 41, No. 2, 69 (1993) • The empirical formula also in: T. Tabata et al., Radiat. Phys. Chem. 54, 11 (1999).

32. Comparison with compiled experimental data and ITS Monte Carlo results (continued) RED SYMBOLS: TABATA

33. Comparison with compiled experimental data and ITS Monte Carlo results (continued) RED SYMBOLS: TABATA

34. Experimental Data on Depth Profiles of Charge Deposition • References • Examples of comparison with ITS results

35. References • Original data and comparison with ETRAN: • T. Tabata, R. Ito, S. Okabe and Y. Fujita, Phys. Rev. B 3, 572 (1971) • Interpolated data and comparison with ITS: • T. Tabata, P. Andreo, K. Shinoda and R. Ito, Nucl. Instr. Meth. B 95, 289 (1995)

36. Examples of comparison with ITS results

37. Examples of comparison with ITS Monte Carlo results (continued) • Minor discrepancies:seen only for Au • Relative deviation d of ITS results of zav from that of experiment: 5 MeV –3.6% 10 MeV –1.8% 20 MeV –2.5%all greater than the probable error of experiment ±1.3%

38. The End