Title: Backscattering of Electrons from 3'2 to 14 MeV: Reflection of Experimental Method and Errors
1Backscattering of Electrons from 3.2 to 14 MeV
Reflection of Experimental Method and Errors
EGS Study Meeting at KEK, August 8, 2007
- Tatsuo Tabata
- Osaka Prefecture University
- and
- Institute for Data Evaluation and Analysis
2Introduction
- T. Tabata, Backscattering of Electrons from 3.2
to 14 MeV. Phys. Rev. 162, 336347 (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
3Introduction (continued)
- As another benchmark, a brief mention is given
of - Our experimental data on depth profiles of charge
deposition
4Backscattering Experimental Method
- Electron beam
- Scattering chamber
- Target and target assembly
- Ionization chamber and measurements
- Background
- Secondary electrons
5Electron beam
- Electron beam produced by
- Linac of the former Radiation Center of Osaka
Prefecture
6Electron beam (continued)
- Analizing magnetCalibrated within an error of
1.1 - Quadrupole magnetsFocused the beam on the
entrance collimater of the scattering chamber 5.5
m away
7Electron 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
8Scattering 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
9Scattering 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 by
cosq cos 20º cosq0
10Scattering chamber (continued)
- Vacuum of the chamber
- Order of 103 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
11Scattering 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 1.92x104 sr
12Targets 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)
13Targets 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
14Ionization 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.
15Ionization 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
16Ionization chamber and measurements (continued)
- Block diagram of measurements
17Ionization chamber and measurements (continued)
- Multiplication factor f 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.
18Ionization 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.
19Ionization chamber and measurements (continued)
- Calibration of f (continued)
- Correction for FC efficiency, for the
backscattering and secondary emission4.18.9
depending on incident energy E0
20Ionization chamber and measurements (continued)
- The calibration curve obtained
21Background
- 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 0.520 of
the signal.
22Background (continued)
- BG for the FC used for calibration
- Measured by inserting an Al plug35 mm long in
the detector collimator - 212 at 160º, increasing with increasing E0
23Secondary 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
24Backscattering Evaluation of Errors
- Possible sources of systematic errors and their
values - Six items
- Total error in backscattering coefficients
- Dressels gross errors
- Comparison with compiled experimental data and
ITS Monte Carlo results
25Possible sources of systematic errors and their
values
- FC efficiency, including the assumption that f
was a function of Eav only 2.98.1 error in f
depending on E0 and Z - Solid angle of detection 1.8 error in ?
- Possible change of d due to electron
bombardment10 error in d - Unmeasured fraction of BG1 error in Ii(q)
26Possible 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 SC to the
target0.5 error in It - Relative indication of the current integrator and
the picoammeter 1.5 error in Ii(q)/It
27Total error in backscattering coefficients
- As shown in Tables I and II of the paper,6.714
depending on E0 and Z - No problem has been found in the evaluation of
errors by the present review.
28Dressels 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.
29Dressels gross errors (continued)
- Dressels experimental arrangement
- Beam monitor located in front of a collimator
- Collimator produced the peripheral halo of the
beam
30Dressels gross errors (continued)
- Beam profile and the peripheral hallo of the beam
Figure, Dressels private communication
31Comparison 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).
32Comparison with compiled experimental data and
ITS Monte Carlo results (continued)
RED SYMBOLS TABATA
33Comparison with compiled experimental data and
ITS Monte Carlo results (continued)
RED SYMBOLS TABATA
34Experimental Data on Depth Profiles of Charge
Deposition
- References
- Examples of comparison with ITS results
35References
- 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)
36Examples of comparison with ITS results
37Examples 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 2.5all greater
than the probable error of experiment 1.3
38The End