Detection Of Radiation With
Semiconductor Diode Detectors
Project by
RAGADEEPIKA PUCHA
Guided by
PROF. L C TRIBEDI
Tata Institute of Fundamental Research
Homi Bhabha Road, Mumbai 400005, India
ABSTRACT
Radioactive materials emit various kinds of radiations. The radiations emitted may be charged like
alpha and beta particles or they may be uncharged like X-rays and gamma rays. These radiations are
detected using different types of detectors. Our main aim is to detect the resolution and efficiency
of such detectors. By properly analysing the spectra obtained for the radiations, we can arrive at the
desired conclusions. For the experimentation, the radioactive sources used in our project are Am 241
and Co57.
INTRODUCTION
Semi-conductor diode detectors are the most commonly used detectors for radiation detection.
These are also known as Solid-state detectors. They have many desirable features which include
compact size, superior energy resolution and relatively fast-timing characteristics. In general, alpha
particles are detected using surface barrier detector, whereas X-rays and gamma rays are detected
using silicon and germanium detectors.
When a junction of p-type and n-type semiconductor is formed, a depletion layer is formed at the
junction. When a ray from the radiation hits the detector, an electron-hole pair is formed due to the
energy deposited by the photon. These conduction electrons and holes migrate and leads to the
pulse formation.
PREPARATION OF TARGETS
Many experiments require the use of target foils of various thicknesses. Hence, it is very important
to know how the desired target foil can be prepared. In general, there are four important ways of
preparing the targets. They are –
1)
2)
3)
4)
Resistive Heating Method
DC Glow Discharge Method
Electron beam Heating Method
Mechanical Rolling Method
Carbon foils, that we used as the targets for the alpha particle detection are prepared using the DC
Glow Discharge Method. This method requires two copper plates, which act as electrodes. A
stainless steel plate coated with a special soap solution, acting as a releasing agent, is placed on the
cathode. After the chamber is evacuated to create a vaccuum of 10-4 mbar, ethylene gas is filled to
maintain a pressure of 0.1 mbar. A potential difference of nearly 2 KV is created across the
electrodes. This results in the dissociation of ethylene molecules into carbon and hydrogen. Carbon
gets collected at the cathode and hydrogen is pumped out. The stainless steel plate is carefully place
in a water bath, where the carbon gets separated from the plate. The set up is calibrated to get a
thickness of 1 µg/cm2 , when the high voltage is passed for 10 seconds.
Figure 1. DC Glow Discharge Method Setup
Figure 2. Glow During the Carbon Foil Preparation
SOME TERMS TO KNOW
1. Resolution of the detector :
The radiations emitted from the radioactive source are obtained as various peaks in
the spectrum. The resolution of the detector is a measure of the width of the peaks. Thinner peaks
denote low resolution whereas wider peaks denote high resolution. It is calculated using the term
“Full Width at Half Maxima (FWHM) (ΔE)” at that point.
Hence, Resolution of the detector at a peak energy E is given by
In general, this term is proportional to
.
.
=
(C – Proportionality constant)
Taking log on both sides log
=
-0.5 log (
+ ln C
Thus, if we draw a graph of resolution at an energy vs square root of the energy in the log scale,
ideally, we should get a straight line of slope -0.5.
2. Efficiency of the detector :
The efficiency of the detector is defined as the no. of quanta of radiation detected
per total no. of quanta of radiation emitted by the source.
The no. of pulses detected of a particular energy is considered as the yield of the particular energy.
The yield Y is hence written as : Y = S.t.B.Є.
, where S is the source strength, B is the
branching ratio of the particular energy of radiation, t is the amount of time for with the readings
were recorded, Є is the efficiency of the detector to detect the particular energy and is the solid
angle.
equation -
Theoretically, the absolute efficiency of the detector is given by the following
Є=
Where µ is the mass attenuation constant of the berillium (every detector has a beryllium window)
and x is the thickness of the beryllium window, which is approximately 25 microns.
And, η is the mass attenuation constant of the material of the detector (Si or Ge) and d is the
thickness of the material in the detector.
3. Relative Efficiency :
In general, the absolute efficiency is difficult to find. Hence, we consider finding of
relative efficiency of the various peaks. We put the efficiency of one of the peaks as 1 and find the
efficiency of the other peaks relative to that peak. This process uses the following equation –
Є1 / Є2 = ( Y1.B2) / ( Y2.B1)
Y1 and Y2 are the yields of the peaks 1 and 2 and is found by finding the area under the peaks
subtracting the base-line. B1 and B2 are the branching ratios for the peaks 1 and 2. If we put Є2 as 1,
we can find Є1 from the above equation after substituting all the required values.
4. Branching Ratio :
Branching ratio for a particular peak is the fraction of the decay of that peak in the
total emission of radiation by the particular source.
EXPERIMENTAL SETUP OF THE DETECTOR
The experiment follows the steps as follows –
Figure 3. Experimental Setup
SURFACE BARRIER DETECTOR
Alpha particles are detected with the help of surface barrier detectors. The only difference
between this detector and the other silicon or germanium detectors is the lack of berillium window
in this detector. This is because the berillium window stops the alpha particles from reaching the
detector as alpha particles looses energy while passing through matter.
AIM – Detect the alpha particles and find the thickness of the target foils used in the experiment.
We assumed here that the foil of thickness 100 µg/cm2 is of correct thickness.
Figure 4. Spectrum obtained for the surface barrier detector
The data is calibrated using the kinetic energies of these two peaks. The resultant is again plotted as
the counts vs kinetic energy curve.
Figure 5. Counts vs KE spectrum
This method uses the concept of stopping power. The stopping power is a function of energy and is
defined as follows. It has units MeV cm2/gm.
S (E) =
Using this concept and the calibration, the thickness of the various foils is calculated. The graph of
calculated thickness vs expected thickness is plotted as follows.
Figure 6. Graph of Calculated Thickness vs Expected Thickness
of the Target Foils
SILICON DETECTOR
Figure 7. Silicon detector (AMPTEK)
We detected X-rays and gamma rays from Am241 and Co57.
Cooling of the detector –
Since the band gap in the semiconductor detectors is very small, some electrons from the valence
band can be thermally excited to the conduction band. These electrons may lead to pulses, which
are not necessary. To avoid this, it is important to cool the detectors. The Silicon detector is cooled
by the Peltier Effect. In simple terms, this is the effect where temperature difference is created due
to the voltage difference.
Counts Vs Channel Number graphs –
Figure 8. Spectrum of Am241
Figure 9. Spectrum of Co57
The spectra are calibrated using two significant peaks of Am241. The same calibration is used for Co57
as well. The peaks used for the calibration are those with Kinetic energies 13.9 KeV and 59.541 KeV.
Counts Vs Kinetic Energy –
Figure 10. Spectrum for Am241
Figure 11. Spectrum for Co57
Using this calibration, we can find the efficiency and resolution of the silicon detector.
Efficiency of the detector –
Figure 12. Plot of Efficiency Vs Kinetic Energy
Resolution of the detector –
Figure 13. Plot of Resolution Vs Kinetic Energy
The slope of this plot is approximately -0.94. It should be ideally -0.5.
GERMANIUM DETECTOR
Figure 14. Germanium detector (CANBERRA)
Cooling of the detector –
Different detectors use different techniques for cooling. This detector is cooled using liquid nitrogen.
Figure 15. Cooling of Germanium detector using Liquid Nitrogen
Counts Vs Channel Number –
Figure 16. Spectrum for Am241
Figure 17. Spectrum for Co57
These are calibrated using the peaks with energies 13.9 KeV and 59.541 KeV of Am 241. The same
calibration is used for Co57 as well.
Counts Vs Kinetic Energy –
Figure 18. Spectrum for Am241
Figure 19. Spectrum for Co57
Using this calibration, we find the efficiency and resolution of the germanium detector.
Efficiency of the Detector –
Figure 20. Plot of Efficiency Vs Kinetic Energy
Resolution of the detector -
Figure 21. Plot of Resolution Vs Kinetic Energy
The slope of this graph is approximately equal to -0.88. It should be ideally equal to -0.5.
CONCLUSION
The thickness of the various target foils has been calculated using Surface barrier
detector. The calculation of efficiency and resolution of the silicon and germanium detectors has
been successfully accomplished.
REFERENCES
1.
2.
3.
4.
NIST ASTAR
NIST XCOM
X-ray data tables
Nuclear data tables from www.nndc.bnl
ACKNOWLEDGEMENTS
I am deeply thankful to Prof. L.C.Tribedi for taking me up as a project student. I thank him
for being patient with me and helping me in learning the subject. I also thanks Siddharth for guiding
me and teaching me the various aspects of the project.
I would also like to thank Tulasiram, Fernandes, Aditya, Arnab, Amarab, Shubadeep and Neelesh for
their support and encouragement. They helped in making my first project a wonderful experience.