LB Thesis DAQ Writeup

The class of detectors used in the photon activation analysis of selenium were high purity germanium detectors. The specific detector used was the GEM40P4 detector from Ortec. This specific detector is cooled with liquid nitrogen to prevent electrons from easily jumping into the conduction band. If the detector is not cooled, then electrons can cross the band gap more easily due to thermal excitations which results in a high level of noise in the detector that will wash out the signal of interest. Once the detector is cold enough, a high voltage can be applied within the detector. For the GEM40P4 detector, the high voltage is set at 4.6kV in a positive bias. After the ionizing radiation passes through the detector, there are 3 main interactions that are of concern. The first interaction is the photoelectric effect. This means that incoming radiation has enough energy to fully liberate an electron creating an electron hole pair. Once the electron hole pair are created within the detector, the electric field created by the applied high voltage will drift the electrons to one terminal and the holes to another which creates a pulse. It should be noted that the size of the pulse is proportional to the energy of the incident photon. This interaction is typically dominant at lower energies.

The second type of gamma-electron interaction we will concern ourselves with is Compton Scattering. This is essentially an elastic collision involving a photon and an electron. As the photon scatters off of the electron, it loses some energy. This basically means that there are photons that can rattle around within the detector. This rattling creates a continuum of energies within the detector. A pulse can be created provided that after Compton Scattering multiple times, the photon liberates an electron via the Photoelectric Effect.

The final interaction is pair production. If the photon has a high enough energy (1.02 MeV) then the photon can change into an electron positron pair. These can produce pulses within the detector. The size of the pulse is proportional to the energy of the electron and the energy of the positron. The positron can also annihilate within the detector, which can create 511 keV peaks.

Once the pulses have been created within the sensitive volume of the detector and amplified by the preamp, they are sent to another amplifier and a discriminator. The amplifier simply further amplifies the signal and the discriminator can be set to reject voltages within a certain range. For example, if the discriminator is set to reject any pulse that is less than 150 mV, then all signals of that voltage or lower will not be registered by the data acquisition system.

Finally the last step is sending the pulse to an analog to digital converter (ADC) which triggers different channels for different voltages. The output of the ADC is sent to a program called MPA. MPA plots the number of counts vs. channel number. At first glance this may not seem useful because we need to translate between channel number and energy, but this can be done easily using calibrated, known radioactive sources. Simply put a source in front of the detector and make a data file with the channel number in one column, the actual energy in another, and the standard deviation from the Gaussian fit around the calibrated source's known energy lines. Usually only a linear fit is needed. The parameters of the linear fit can translate from channel number into energy.

Another property of the detector that must be addressed is the efficiency. Unfortunately the detector does not operate at 100% efficiency. To find the efficiency one can use calibrated sources with a known activity and reference date to compute the theoretical rate of decay of the source. One this has been calculated, use MPA (or another program that can plot the spectra, such as ROOT) to find the actual number of counts seen by the detector. The ratio of these two numbers will give you the efficiency. There are two types of efficiency that must be taken into account. The first type is the energy efficiency. Higher energy photons don't create as many events that register within the detector. The second type is the geometric efficiency. As the sample moves farther away from the detector, less photons will actually pass through the detector since the detector is a relatively small cylinder. The efficiency for the GEM40P4 detector can be found below.

LB May 2017 Det A Efficiency

Below are the physical specifications of the GEM40P4 detector found on Ortec's website.