LB Thesis DAQ Writeup
Identify models of adc
The class of detectors used in the photon activation analysis of selenium were high purity germanium detectors. The specific detector used was the GEM40P4 high purity germanium 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 means that photons can enter the crystal, deposit energy via multiple Compton events, and leave the crystal. This energy deposition via Compton Scattering creates a continuum of energies within the detector. This continuum doesn't present itself in the entire spectrum. There is a maximum amount of energy a photon can transfer to the electron. This results from a cosine dependence in the transferred energy. The maximum energy a photon can deposit will be when it back-scatters off of the electron. As a result, the Compton Continuum has a sharp edge at the maximum value of transferred energy. A sample spectrum is shown below.
[Show Picture Of Spectrum]
A signal can also be created if a photon Compton Scatters multiple times, then 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 pre-amp, they are sent to a post-amp and an ADC with an internal constant fraction discriminator. The amplifier simply further amplifies the signal and the discriminator can be set to reject voltages within a certain range.
The analog to digital converter uses an internal constant fraction discriminator, which takes an input signal and duplicates it, and scales it by some factor. The duplicated signal is then delayed for some amount of time that is less than the rise time of the signal and a "zero crossing point" is searched for. If the crossing point is found, then a pulse is created, but if no crossing point is found, then the signal is rejected. For the purposes of this experiment the internal CFD was set to reject signals that result is a measurement of 40keV or lower.
Finally the last step is sending the pulse to a peak sensing analog to digital converter (ADC). An analog pulse is created by the detector and when the gate is open, the data acquisition will take place. Assuming the gate is open, the analog pulse is sent through a differentiating circuit, which has an output that is proportional to the rate of change of the signal. Once the circuit finds a maximum, the voltage is held constant and the peak voltage is maintained by a capacitor. If a larger peak appears while the gate is open, the capacitor will charge more to compensate for the larger peak. A comparator is used to check if the analog signals are greater than the previous peak. After the gate closes, the capacitor will discharge and another gate opens. The gate remains open for as long as the capacitor is discharging. This means that the duration the gate is open is directly proportional to the voltage across the discharging capacitor.This information is assigned a numerical address which is unique to the voltage across the capacitor.
A wire diagram is shown below:
The model of the post amplifier is the ORTEC model 672 Spectroscopy amplifier. The coarse gain was set to 20 and the fine gain was set to 42. The power supply used was an ORTEC 659 5kV Radiation Detector Detector Bias Supply, which was set to 4.5 kV. The model of the ADC used was the Canberra Model 8701, which is a Wilkinson ADC which samples at 100MHz. The integral non-linearity is less than 0.025% over the top 99.5% of the selected range, while the differential non-linearity is less than 0.7% over the top 99.5% of the selected range.
The output of the ADC written to a binary "list" file by a program called MPA, which stands for Multiparameter Data Acquisition System. MPA will also plot the contents of the list file as 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. Once 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.
| Model No. | Endcap Size (mm) | CFG | Serial No. | Relative Eff @ 1.33 MeV (%) | FWHM @ 1.33 MeV | Ratio FW1M FWHM | Ratio FW02M FWHM | Peak to Peak Compton | FWHM @ 14 keV | FWHM @ 5.9 keV | FWHM @ 122 keV | Crystal Diameter (mm) | Crystal Length (mm) |
| GEM40P4-83-SMP-ST | 83 | PT | TP43076A | 44 | 1.93 | 2.0 | 2.9 | 60 | 700 | 66.7 | 49.2 |