PHYS291 Project documentation


I study nuclearphysics at the dept. of physics, University of Bergen.
This is the documentation for a project in Phys291. Phys291 is an UIB-course which aimes to give an introduction to C++ and Root.
For my project topic I chose to look at how alpha's, beta's and gamma's form clusters when they deposite radiation.

Program code

The source codes and the data files are available on the code page. The code files are commented, and further descriptions are provided under results below. For the collecting of data a detector named ALPIDE was used. The Alpide detector is a SiPM, with the following parameters


It collects a hit in the following way:


When the particle hits a pixel in the detector it ionizes the medium according to the Bethe-Bloch formula. When this happens the electrons starts to drift towards the collection diode which gives the signal.

Fake-Hit Rate


To estimate the fake-hit rate, the chips are read out in a sequence of consecutive events without providing an external stimulus like a radioactive source, beam or external pulsing. The average fake-hit rate is calculated according to:

with N_Hits denoting the number of hits detected by the number of pixels under study N_Pixels and the number of recoreded events N_Events.

For these measurements three different bias voltages where used. This was done to analyze which bias voltage that would produce the best results. By applying a bias voltage one can increase the number of particles detected, but at the cost of some noise. Here is the hit maps for the background.

As expected the noise increases when a back bias voltage is applied. The line at that is observed occurs because there is a line of dead pixels in the detector at these points.

In turn a gamma ray source was placed on the detector and measurements were taken with the different revere back-bias voltages.

Gamma source- reverse back bias -6V



Most of the radiation from the source is deposited at the point where the source was placed. There are some signals further away from the source since the gammas are emitted in different directions and can travel for some distance before they are detected. There are also some noise which corresponds to some of the hits further away from the source because of the bias voltage.
Zoomed


Gamma source - reverse back bias -3V



Most of the radiation from the source is deposited at the point where the source was placed. There are some signals further away from the source since the gammas are emitted in different directions and can travel for some distance before they are detected. There are also some noise which corresponds to some of the hits further away from the source because of the bias voltage. As can be observed there are less noise in this picture as expecded due to the reduced bias voltage compared to the source at 6V.
Zoomed

Gamma source - reverse back bias 0V



Most of the radiation from the source is deposited at the point where the source was placed. There are some signals further away from the source since the gammas are emitted in different directions and can travel for some distance before they are detected. Here there are less noise, due to the fact that there aren't any bias voltage applied.

The source used was Fe55. This is a radioactive isotope of iron which decays by electron capture to Mn55 by sending out characteristic x-rays with energy 5.9 keV and 6.49 keV. This process has a half-life of 2.737 years.

_ Comparison of fe55 clusters



Comparison of clusters from fe55 source at different bias voltages. Green dots are at 6V back bias. Blue dots are at 3V back bias. Red dots are at 0V back bias.

As can be observed from the plot above, the 6V back bias provides the strongest signal, but also the most noise. This decreases as the bias voltage is decreased.


Hitrate for different voltages




Hitrates for different voltages for Y and X pixels. It can be observed that with an increased voltage the hitrate goes up. The different ratioes are as follows:
For Y: 3V/0V=1.17798, 6V/0V=1.23854.
For X: 3V/0V=1.12552, 6V/0V=2060.23.

The high ratio between 6V/0V for the X seems to come from an enormous amount of counts in the bin numbers:
Bin # 491 has 2 million hits
Bin # 504 has 1.7 million hits
Bin # 505 has 198 000 hits.
If these are exluded the 6V/0V=0.005 which is obviously far to low.
if the bins 491 and 504 are exluded then 6V/0V=103.6 which is to high when looking at the plot and comparing to the values found for the Y pixels.


Slope of fe55 sources




As can be observed there aren't much of a correlation between the slope and the different voltages as they share a simmilar form.

Alpha source - reverse back bias -3V.


Most of the radiation is deposited where the source was placed. We have a less dense signal compared to the Gamma source since this process has a longer half-life. There are also less signals further out since the Alpha-particle isn't able to travel far.

The source used was Am241. This is a radioactive isotope of americium which decays by alpha decay to Np237 by sending out a characteristic alpha with an energy of 5.4 Mev. This process has a half-life of 432.2 years.

_ Beta source - reverse back bias -3V.


Most of the radiation is deposited underneath the source. Here there are hits all over the detector as the electron travels in all directions and can travel a fair distance before being detected.

The source used was Sr90. This is a radioactive isotpe of strontium which decays by beta minus decay into Y90 by sending out an electron with a mean energy of 0.546 MeV. This process has a half-life of 28.49 years. The Y90 then undergoes another beta minus decay with a half-life of 64 hours into Zr90. Because of the large difference in half-life the 90Sr/Y is almost a pure beta particle source with the energy spectre of the electron yielding an energy release of 2.2 MeV.

Conclusion


Before I took this course I had done little programming and not any C++. After working on my project I feel that I now have some tools to analyze my data. This is a good way to manipulating and visualising the data and I will make use of ROOT in my future.