Installing a Muon Rejection System for Gamma Spectroscopy in Hydrogen Detection at Tandem Laboratory (Muren)

Sammanfattning

When quantifying hydrogen using the 1H(15N,αγ)12C nuclear reaction background radiation due to cosmic ray muons is problematic. A passive filter such as encasing the gamma ray detector in a thick layer of lead will not be sufficient to stop the muons and the detector used to measure the amount of gamma rays cannot distinguish between incoming muons and gamma rays. This is especially problematic considering that the muons will appear in the same part of the energy spectra as the radiation used to quantify the hydrogen. For low hydrogen concentration the number of muons detected is of the same order as the gamma rays and the gamma rays are indistinguishable from the background. Therefore solving this problem would lead to the ability to quantify lower concentrations of hydrogen than before. The proposed solution is a type of active filter. In order to accomplish this a type of detector called scintillators were used where one was placed above and one below the gamma ray detector. The muons are detected by scintillators generating a light pulse which is converted to a current using photomultipliers. The voltage signal from the photomultipliers were amplified and then passed through a discriminator to filter out electrical noise. After the discriminator the signal was passed through a coincidence unit that only generated a signal if the signals from the two photomultipliers overlap within 100 ns. The signal from the coincidence unit was then passed into a gate generator where the signal was modified to fit the width of the signal from the gamma ray detector in order to effectively block the signal being fed into the computed when a muon passes through the detector. The result of this was a muon veto that should be able to reduce the background radiation due to cosmic ray muons. A working muon detector was built and was tested for vetoing the gamma ray signal. Even though the detector itself showed promising results it was more difficult to get the actual muon veto to work properly. Even though the setup was seemingly working when observing the veto and the gamma ray signal on an oscilloscope the setup was unsuccessful in producing a noticeable difference when comparing the energy spectra on the computer with and without the muon veto active. The reason for this was after much reworking attributed to compromises due to the available equipment.