The muon detector we want to realize is based on a plastic scintillator crystal coupled to a photomultiplier. This tool completes the coincidence detector described in the post : Cosmic Rays and Coincidence Detector.
Plastic crystal and PMT have been inserted inside a cylindrical metal housing. Inside the container it has been placed also the high voltage driver for the photomultiplier tube. The signal from the anode of the PMT is picked up by means of a decoupling capacitor and sent to a BNC connector on the container lid. On the lid is also placed the connector for the low voltage power supply at 5V. In the images below you can see the finished detector closed and opened.
This detector is sensitive also to background radioactivity but you can easily select the pulses generated by cosmic muons since the latter have an amplitude much greater than the ones produced by background gamma radiation. To optimize the yield for the muon particles the plastic scintillator type BC412 has been chosen. It is particularly suited to detect charged particles such as electrons or muons. The area of the scintillator, 119 cm2 has been chosen wide in order to obtain a high number of events per second due to cosmic muons, also the thickness of the scintillator has been chosen quite big, 114 mm, so as to increase the stopping power and thus increase the likelihood that the muons are slowed down and come to a rest inside the scintillator.
The photomultiplier tube is driven by means of a PMT adapter Theremino placed inside the metal container. Actually it is used only the high voltage generation part. The signal is taken from the coupling capacitor C8, while the entire part of “pulse shaping” has not been assembled. The HV driver power supply is via 5V low voltage, which can be easily provided by a series of 4 1,5V batteries, the voltage can reduced a bit with a diode connected in series.
The typical high-energy muon passes right through the scintillator, but in doing so, it causes some ionization, and deposits about 50 MeV of its energy in the scintillator. And some fraction of that energy gets converted to photons of light, and some fraction of that light reaches a photomultiplier tube, which converts the brief flash of light to a detectable pulse. Fortunately, the much more frequent events due to background radiation of the earth from ambient beta and gamma rays, have an initial energy of 1 MeV or less. A discriminator easily filters out the weak light pulses they create.
by adjusting the supply voltage of the photomultiplier to around 1000V, the typical pulse produced by a muon passing through the scintillator crystal has a width of about 200-400mv; adjusting the threshold to trigger the oscilloscope to 200mv automatically selects only the pulses produced by the muons.
The pulses produced by the PMT may also be acquired by means of a transimpedance amplifier, like that one shown in the following scheme.
The advantage is that the load resistor may be set for example at 50Ω so as to reduce the time constant of the circuit, without sacrificing the amplification obtained by the resistor R2 : Vout = R2 * Ipmt. The figure below shows a pulse acquired with a TIA. We see how the pulse FWHM is around 50ns.
The pulses produced by the scintillation detector may be conveniently displayed by a software MCA. To do this connect the BNC output of the detector with the “pulse shaper” PMT adapter Theremino, its audio output is then connected to a PC running the software Theremino MCA. The gamma spectrum obtained shows a peak at high energies (tens of MeV) , higher than the peaks of the normal gamma energies. This peak corresponds to the energy deposited by muons interacting with the plastic scintillator. For a muon with vertical path the maximum deposited energy is about 50 MeV. Of course the peak you get does not match the actual energy of the muon, since muons with higher energy pass through the scintillator crystal and continue their path, while muons with lower energy are stopped within the crystal and undergo the consequent decay.
Gamma Spectrometer Measures
The cosmic ray detection can also be done with a normal gamma spectrometer with the classic NaI (Tl) scintillator, for good results, however, it is necessary to use a fairly large crystal and collect the data for a long time (12-24 hours). We used a 63 x 63 mm crystal sensor positioned vertically and powered at 750 V. The description of the device is in the post: DIY Gamma Spectrometry. We utilized the software application BecqMoni which is suitable to make countings.
The images below show the background gamma spectrum after a few hours of detection. After the peak at 2600 KeV of Tl 208 (from the Thorium decay chain) the number of counts is practically zero and then there is a pronounced peak of about 5000 KeV. This peak corresponds to the energy deposited by the muons interacting with the scintillator. For a vertical path the maximum deposited energy should actually be around 20 to 30 MeV, but the nonlinear crystal response moves this peak to much lower energies.
By utilizing the BecqMoni ROI feature, we made the count on the muon peak, obtaining a value of 0.82 CPS – 49.2 CPM, as shown in the figure below.
Given a normal particles flux of 0,01 particles/s cm2 steradiant, The flux of particles reaching the detector (at sea level) should have the following value:
Detector Surface = 31,2 cm2
31,2 cm2 x 0,01 particles/s cm2 = 0,312 particles/s = 18,7 cpm
Actually the measured value is higher because the plastic scintillator is achieved by cosmic particles also through the sides. Also measurements were made at altitudes above sea level.
For cosmic ray flux measurements the signal produced by PMT is sent to a discriminator circuit which produces an output TTL logic level 1 when the signal exceeds a chosen threshold : in this way you select only the pulses of high amplitude produced by cosmic muons. By decreasing the threshold it is possible to detect also the normal environmental radioactivity. As a further screen for environmental radioactivity the scintillator has been placed on top of a lead brick with a thickness of 50mm.
The flow of particles that reach the detector (at sea level) should have the following value :
Detector Surface = 119 cm2
119 cm2 x 0,01 muons/s cm2 = 1,19 muons/s = 71 cpm
Actually, even in this case, the measured value is higher because the plastic scintillator is achieved by cosmic particles also through the sides. Also measurements were made at altitudes above sea level.
The table and graph below shows the obtained results :
Pdf document with the description of the project : RilevatoreMuoni_ENG