# Response of semiconductor detectors: comparison between experimental measurements and Monte Carlo simulation

Marie-Christine Lépy, Johann Plagnard

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Marie-Christine Lépy, Johann Plagnard. Response of semiconductor detectors: comparison between experimental measurements and Monte Carlo simulation. EXRS 2008 - 13th European Conference on X-ray Spectrometry, Jun 2008, Cavtat, Croatia. cea-05457371

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# Response function of semiconductor detector : comparison between experimental measurements and Monte Carlo simulation

Marie-Christine Lépy, Johann Plagnard

Laboratoire National Henri Becquerel, CEA-LNE, CEA Saclay,
F-91191, Gif-sur-Yvette CEDEX, France
# Presentation

* Introduction

* Experimental arrangement

- Monochromatic X-ray source (SOLEX)

- GeHP detector characterized using SOLEX

* Monte Carlo simulation

* Comparison with spectra obtained with monochromatic beam

* Summary and perspectives
# Introduction

* Detection limit depends on the resolution and the background

* Peak-to-background ratio : 1000 to 10000

* Background not linear : smooth shapes

* Examination on the structure of the background

* Better knowledge -> improved spectrum processing
* Monochromatic X-ray source in the 1-20 keV energy range

<figure><img src="image_2.png" /><figcaption>C. Bonnelle et al.
Nuclear Instrum. Methods in Phys. Res. A 516, 594-601 (2004)</figcaption></figure>

<figure><img src="image_3.png" /></figure>

Vacuum chamber

X-ray tube

Dispersive crystal

Monochromatic beam in a
constant direction
<figure><img src="image_4.png" /></figure>

Two positions (30° rotation)

Allow successive measurements to perform beam monitoring
<figure><img src="image_5.png" /></figure>

J. Plagnard, C. Bobin, M.C. Lépy
X-Ray Spectrometry, 36 (2007) 191-198

## Supplier information :

- Germanium crystal :

- surface10 mm²

- thickness : 4 mm

- 10 µm Be window

- Al : 100 nm

- Ni : 50 nm

- Ge dead layer : 50 nm

FWHM = 115 eV at 5.9 keV

Peak(5.9 keV)-to-background (1 keV) ratio > 10 000

## Detector characterized using SOLEX

- Tuning of incident energy -> Scanning around the binding energy
- Monitoring using a reference detector
# Energy scanning

<figure><img src="image_6.png" /></figure>

$$A = \frac{I_{\text{below}}}{I_{\text{above}}} = \frac{\exp(-\mu_{\text{below}} \cdot x)}{\exp(-\mu_{\text{above}} \cdot x)}$$

X = Absorbing layer

μ = linear attenuation coefficient
below and above the binding
energy

## Al K edge scanning

$$X = \frac{\ln(A)}{\mu_{\text{above}} - \mu_{\text{below}}}$$

<figure><img src="image_7.png" /></figure>

## Ge L3 edge scanning
Thickness results

<table><thead><tr><th rowspan="2">Layer</th><th rowspan="2">Binding energy (keV)</th><th colspan="2">Thickness (nm)</th></tr><tr><th>Specification</th><th>Measurement</th></tr></thead><tbody><tr><td>Al</td><td>1.56 (K)</td><td>100</td><td>113 (18)</td></tr><tr><td>Ni</td><td>8.33 (K)</td><td>50</td><td>51 (20)</td></tr><tr><td>Ge<br>PAL</td><td>11.10 (K)</td><td></td><td>1270 (40)</td></tr><tr><td>Ge<br>dead layer</td><td>1.22 (L3)</td><td>50</td><td>12 (6)</td></tr></tbody></table>
# Monte Carlo simulation

* PENELOPE code (F. Salvat - Univ. Barcelona):
  PENetration and Energy LOss of Positrons and Electrons

* Energy range : a few $10^2$ eV to 1 GeV

* Atomic relaxation : emission of characteristic radiation and Auger electrons (including Coster-Kronig transitions) resulting from vacancies produced in K, L and M shells.
# Geometry

* Parallel monochromatic beam impinging on slices of successive materials :

- Al and Ni layers

- Ge dead layer

- Ge crystal

(The beryllium window and boron layer were not considered)

<figure><img src="image_8.png" /></figure>

Monochromatic
beam

<figure><img src="image_9.png" /></figure>

* Only energy deposition : no further assumption on pair creation and charge collection
# One incident photon : 7 different ways for energy absorption in Ge

## First interaction in Ni or Ge : mainly by photoelectric effect

-> photoelectron (红箭头) + vacancy

-> rearrangement with X-ray (蓝箭头) or Auger electron (黑箭头)

<figure><img src="image_10.png" /></figure>
# Primary interaction in Al

* Al K binding energy = 1.56 keV

* Incident photons 1 to 15 keV

* > 98% transmission, except in the 1.6 to 5 keV energy region

* 11.4 % absorption at 1.6 keV

* Low backscattering (< 0,1 %)

<figure><img src="image_11.png" /></figure>

* Transmitted secondary particles

* Electrons $10^{-3}$

* Photons $10^{-6}$ to $10^{-3}$

<figure><img src="image_12.png" /></figure>
# Primary interaction in Ni

* Ni K binding energy = 8.33 keV

* Incident photons : 5 to 12 keV

* > 98% transmission

* 0.2 to 1.4 % absorption

* Backscattering:

- < 0.01 % below 8.33 keV

- about 0.1 % above 8.33 keV

* Transmitted secondary particles:

* Electrons $10^{-3}$

* Photons $10^{-5}$ to $10^{-3}$

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>
# Transmitted particles (Ni)

Incident energy = 8 keV

<figure><img src="image_15.png" /><figcaption>Transmitted electrons through Ni (E=8 keV)</figcaption></figure>

<figure><img src="image_16.png" /></figure>

Incident energy = 8.5 keV

<figure><img src="image_17.png" /><figcaption>Transmitted electrons through Ni (E=8.5 keV)</figcaption></figure>

<figure><img src="image_18.png" /></figure>
# Primary interaction in Ge dead layer

* Ge K binding energy = 11.10 keV

* Incident photons 1 to 15 keV

* > 99% transmission

* 0.1 to 1 % absorption

* low backscattering ($< 0.1 \%$)

<figure><img src="image_19.png" /></figure>

*Transmitted secondary particles

* Electrons $10^{-3}$

* Photons $10^{-7}$ to $10^{-4}$

<figure><img src="image_20.png" /></figure>
# Primary interaction in Ge active layer

* Ge K binding energy = 11.10 keV

* Incident photons 1 to 15 keV

* > 99% absorption

* No transmission

<figure><img src="image_21.png" /></figure>

* Backscattered secondary particles:

* Electrons $10^{-3}$

* Photons $10^{-6}$ to $10^{-1}$

* Intense Ge K escape for $E > 11.1$ keV

<figure><img src="image_22.png" /></figure>
# Deposited energy in Ge from secondary particles from Ni

Example with incident energy = 8.5 keV

<figure><img src="image_23.png" /></figure>

Mainly Ni K photons

<figure><img src="image_24.png" /></figure>

Deposited energy by electrons from Ni (E=8.5 keV)

0 -> Ephotoelectron

0 -> EAuger electrons
Comparison around the Ni K binding energy

<figure><img src="image_25.png" /><figcaption>Deposited energy (E = 8.5 keV)</figcaption></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

a)

<figure><img src="image_28.png" /></figure>

b)
# Comparison around the Ge K binding energy

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>
# Summary

Use of a tunable monochromatic X-ray source to characterize HPGe detector

-> geometrical parameters to feed Monte Carlo simulations

Coherent results between simulation and experimental data

Demonstration of the influence of electrons signature in the background
on the left side of the full-energy peak

Only qualitative characterization in term of deposited energy.

This study does not take into account the charge carriers creation and diffusion nor processing of the resulting signal.
Next step :

« Metrology » beam line on a bending magnet : 3 branches to cover the energy range from a few eV to 28 keV

The « hard X-rays » branch will provide monochromatic radiation in the 100 eV to 28 keV energy range : 3 monochromators : double Si crystal, or mirror + grating

<figure><img src="image_33.png" /></figure>

Reference detector for characterization and calibration of secondary detectors (2 positions)
# Present state

<figure><img src="image_34.png" /><figcaption>X-UV Monochromator</figcaption></figure>

First photons expected in late 2008 -
Usable from spring 2009 !!!

<figure><img src="image_35.png" /><figcaption>Hard X-rays monochromator</figcaption></figure>