Backscattering of Electrons from 3'2 to 14 MeV: Reflection of Experimental Method and Errors

1
Backscattering 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

2
Introduction

• 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

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

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Electron 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

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

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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 by
  cosq cos 20º cosq0

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Scattering 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

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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 1.92x104 sr

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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)

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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

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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.

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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

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Ionization chamber and measurements (continued)

• Block diagram of measurements

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Ionization 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.

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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.

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Ionization 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

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Ionization chamber and measurements (continued)

• The calibration curve obtained

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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 0.520 of
  the signal.

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Background (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

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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

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Backscattering 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

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Possible 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)

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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 SC to the
  target0.5 error in It
• Relative indication of the current integrator and
  the picoammeter 1.5 error in Ii(q)/It

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Total 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.

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Dressels 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.

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Dressels gross errors (continued)

• Dressels experimental arrangement
• Beam monitor located in front of a collimator
• Collimator produced the peripheral halo of the
  beam

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Dressels gross errors (continued)

• Beam profile and the peripheral hallo of the beam

Figure, Dressels private communication
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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).

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Comparison with compiled experimental data and
ITS Monte Carlo results (continued)
RED SYMBOLS TABATA
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Comparison with compiled experimental data and
ITS Monte Carlo results (continued)
RED SYMBOLS TABATA
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Experimental Data on Depth Profiles of Charge
Deposition

• References
• Examples of comparison with ITS results

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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)

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Examples of comparison with ITS results
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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 2.5all greater
  than the probable error of experiment 1.3

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The End