A project report on Characterization of

semiconductor diode detectors –

efficiency and resolution.

Submitted by

KARUKA SIDDARTH REDDY

Guided by

PROF. L C TRIBEDI

टाटा मलभत अनसधान ससथान

Tata Institute of Fundamental Research

Homi Bhabha Road, Mumbai 400005, India

Abstract

Generally semiconductor diode detectors are employed in detecting various radiations emitted by a radioactive source. With an aim to find out the efficiency and resolution of such detectors spectra of various radioactive sources ( Am241 and Co57 ) are collected. Analysis of this spectrum gives us the desired result. Also the source strength of Am241 is found out experimentally in order to get the intrinsic efficiency of detector. Source strength is measured using the Surface Barrier detector.

Introduction

Various types of detectors are used in detection of radiation emitted by radioactive sources. Among these, semiconductor diode detectors are widely used because of their advantages such as superior energy resolution, compact size, and relatively fast timing characteristics over the remaining. Every detector has range of energies in which they detect the radiations effectively. Thus to obtain a satisfactory output of a radiation it is necessary to know the type of detector to be employed, which in turn gives the best result in that range of energies of the radiation. Thus it becomes essential to know the variation of efficiency with energy, which is the main essence of this project.

When a p-type semiconductor and an n-type semiconductor are sandwiched, a depletion layer is formed at the junction. When an emitted ray strikes the detector part, it deposits energy and equal number of conduction electrons and holes are formed. The bias setup for the detector ensures that electric field is present in the region where the electron-hole pairs are created. Thus these conduction electrons and holes migrate and thus constitute to the pulse formation. Considering the point of bias setup, generally the junction will readily conduct current when voltage is applied in the forward direction, but it will conduct very little current when biased in the reverse direction.

Efficiency

Absolute efficiency is defined as number of pulses recorded per number of radiation quanta emitted by source.

The number of counts recorded Y, is given by

Y = S.t.B.(Ω/4π).Є

S is the source strength, t is live time, B is branching ratio, Ω is the solid angle of the detector seen from the actual source position, and Є is the efficiency of the detector.

Relative efficiency

It is easier to find out the relative efficiency because the source strength, solid angle and time remain constant and would cancel out each other when relative things are taken. Thus,

Є₁ / Є₂ = (Y₁B₂) / (Y₂B₁)

When ϵ₂ is taken to be 1, ϵ₁ can thus be calculated and thus relative efficiency of the detector is obtained. Total number of counts, Y, is obtained by the area under the desired peak, and the corresponding branching ratios are used.

Resolution

The radiations detected produces peaks in the spectrum. The width of the peaks is determined by the resolution of the detector. High resolution enables to separate lines that are close to each other.

The energy resolution of the detector is conventionally defined as the full width at half maximum (FWHM) divided by the location of the peak.

Hence resolution is given by ∆𝐸

𝐸

∆𝐸

𝐸 ∝

1

𝐸

ln ∆𝐸

𝐸 ∝ ln

1

𝐸

ln ∆𝐸

𝐸 = ln

1

𝐸 + ln𝐶

ln ∆𝐸

𝐸 = −0.5 ln 𝐸 + ln 𝐶

Thus when we take a plot of resolution versus energy, in log scale, ideally the slope of the line is -0.5

Experimental section

A rough diagram of the experimental setup used is shown below.

Semiconductor diode detectors such as Germanium (Canberra), Silicon (Amptek) and surface barrier detectors are used for detecting the radiations of Am241 and Co57.

Cooling with liquid nitrogen

Most of the detectors are cooled with liquid nitrogen. This is done to ensure that there is no leakage current flowing through. Since the band gap in the semiconductors is very small it is possible for the thermal energy to excite the electrons from valence band to conduction band which may lead to leakage current. Such current does not constitute to the radiation detected and thus gives false interpretations. Hence the thermal energy is brought down by cooling with liquid nitrogen (-196°C) and thus the leakage current.

Cooling Germanium semiconductor detector with liquid nitrogen:

Silicon detector used is also shown below

All the detectors are to be cooled necessarily but different detectors may use different techniques to be cooled. Silicon detector is not cooled using the liquid nitrogen and Peltier cooling takes place in this case. Peltier cooling is based upon the Peltier effect (Thermal difference is created when there is a voltage difference).

Depending on the detector used, different voltages are applied to different detectors. For the Germanium detector used the voltage setup is (-)550V.

The detector is then connected to a preamplifier. Usually it is connected with a short wire in order to decrease the extent of noise. Function of preamplifier is to match impedance of the detector and amplifier which allows the signal transmission effective. It is then connected to an amplifier. Amplifier is used for adjusting gain which allows the signals obtained to match with the calibration of the MCA used.

The output is received by MCA (Multi Channel Analyzer) and then the data is recorded output being a spectrum of the received radiations. The output is shown in the following section.

Germanium (Canberra)

Americium241:

Cobalt57:

Silicon (Amptek)

Americium241:

Cobalt57:

The above obtained are turned out into energy scales by calibrating the obtained spectrum using two known lines, preferably the lines at the either ends to minimize the error in calibration.

Counts versus Energy

Germanium (Canberra)

Americium241

It is calibrated using the peaks 59.54keV and 13.9keV which occur at the row number 3352 and 787 respectively.

Cobalt57

It is calibrated using the peaks 136.47keV and 6.4keV which occur at the row number 7676 and 360 respectively.

Silicon (Amptek)

Americium241

It is calibrated using the peaks 59.54keV and 13.8keV which occur at the row number 2381 and 556 respectively.

Cobalt57

It is calibrated using the peaks 136.47keV and 14.412keV which occur at the row number 5458 and 576 respectively.

Relative efficiency curves

Germanium (Canberra)

Americium241

Here the efficiency is taken to be 1 at 59.52keV and the efficiency at other energy values is calculated accordingly.

Cobalt57

Here the efficiency is taken to be 1 at 122.06keV and the efficiency at other energy values is calculated accordingly. Also the efficiencies thus obtained are normalized accordingly by one point normalization at 122.06keV.

Silicon (Amptek)

Americium241

Here the efficiency is taken to be 1 at 13.86keV and the efficiency at other energy values is calculated accordingly. Also the efficiencies thus obtained are normalized accordingly by one point normalization at 13.86keV.

Cobalt57

Here the efficiency is taken to be 1 at 7.011keV and the efficiency at other energy values is calculated accordingly.

Resolution

Germanium (Canberra)

Americium241

Cobalt57

Silicon (Amptek)

Americium241

Cobalt57

Source strength

Source strength of Am241 is detected by using a surface barrier detector which detects the alpha particles emitted by the source and by analyzing this we can calculate the source strength of the source as alpha particle emission is the only mode of decay of Am241.

Block diagram of the setup used is shown below.

A rough image of the experimental setup is shown below.

The bias used is 50V and the spectrum obtained is shown below.

Number of radiations detected by the detector is given by

𝑌 = 𝑆 ∗ 𝑡 ∗ ∈ ∗ Ω

4π

Hence source strength S is given by

𝑆 =𝑌∗4𝜋

𝑡∗∈∗Ω

Efficiency of the detector is approximately 1 at 5.4Mev and hence the factor ∈ doesn’t play a major role. Solid angle Ω depends on the distance of the detector from the source and also on the surface of the detector. The values are t = 3368seconds, Ω = 0.0029.

Taking all these factors into account, the obtained Source strength is roughly 1.1*10⁶ Bq i.e. 30µCi

This source strength is used in calculating the intrinsic efficiency of Si(Li) detector.

A picture of the Si(Li) detector used is shown below.

If required, this detector can also be operated in a windowless mode after vaccum is created. The source is placed at a distance of 90mm from the Beryllium window of the detector. The bias setup is (-)500V. The spectrum obtained is shown below.

Area under peaks is calculated and this gives the number of counts in that energy range. Accordingly the intrinsic efficiency of the detector at various energy values can be calculated using the relation

Є = (Y * 4π) / (t * Ω * S * B)

where B is the emission probability.

The values thus obtained are shown in a plot below.

The emission probabilities used are from the following table, taken from Nuclear Instruments and Methods in Physics Research A312 (1992) 323-333.

At a glance

The efficiencies obtained using different sources (Am241 and Co57) for the same detector are shown in the same plot below to give a better overview of the detector efficiency at different energy values.

Germanium (CANBERRA)

Silicon (AMPTEK)

Conclusion

From the results obtained the source strength of Am241 which is being used is now known to be 30µCi approximately and also the variation of efficiency with energy of Germanium and Silicon semiconductor diode detectors is known. Also the variation of absolute efficiency with energy of the Si(Li) detector is known.

References

1. C.J. Bland, J. Morel, E. Etcheverry and M.C. Lepy, Determination of 239Pu and 24'Am LX-ray

intensities using a Simplex method for fitting peaks, Nuclear Instruments and Methods in Physics Research A312 (1992) 323-333.

2. M S Basunia, Nuclear Data Sheets 107, 3323 (2006). 3. M R Bhat, Nuclear Data Sheets 85, 415 (1998). 4. NIST XCOM.

Acknowledgements

I am deeply thankful to Prof. L C Tribedi for taking me under his guidance. Siddarth and Aditya were very patient with me and helped to learn and guided me in every aspect. I would also like to thank Thulasiram, Fernandes, and also Arnab khan, Amarabha Banerjee and Neelesh for their help and support, making my first project and the laboratory work an enjoyable experience. I thank all people I worked with, listened to, and chatted with here at TIFR.