Some time ago in our laboratory we built a cosmic ray detector based on plastic scintillator and PMT : Scintillation Detector for Cosmic Muons. The goal was to experiment with this type of detectors and make cosmic rays flux measurements at various altitudes : Cosmic Rays on the Dolomites. Subsequently the same instrument was used to have evidence of the muon’s decay and to measure its half-life : Cosmic Ray Muons and Muon Lifetime.
Our “scintillator can” really did a great job !
At this point we decided to use the detector to continuously monitor the cosmic rays flux and try to relate it to other parameters such as atmospheric pressure, intensity of the geomagnetic field and solar activity.
It is known that the cosmic rays flux (mainly muons) that reach the earth’s surface depends on many factors, including the density of the atmosphere and the geomagnetic field. With increasing density – and therefore pressure – the absorption of the particles by the gases that make up the atmosphere increases; in addition, solar activity and the geomagnetic field (influenced in turn by solar activity) deflect the trajectories of primary cosmic rays and therefore act as a screen.
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 images below show both the scintillator and the coupling with the PMT.
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.
The signal produced by the PMT, impulsive and of limited duration, is sent to a pre-amplifier which appropriately amplifies the signal, bringing its duration to a few tens of micro-seconds. The device was described in the following post : Micod PMT Universal Amplifier. The typical pulse, produced by a cosmic ray, output from the amplifier has an amplitude greater than 1V and a duration of less than 50μsec. The image below shows the amplifier and the complete instrument.
The signal produced by the amplifier is sent to an analog input of the PSoC card (shown in the cover image of the post). The signal is processed to produce an impulse which is then sent to a counter. The diagram below shows the programming logic of the PSoC : the signal is sent to a comparator with hysteresis with a SW programmable threshold (currently set at 1V) which produces a rising edge converted into a 1 μsec pulse by the D-type flip-flop , the pulse is then sent to the counter for pulse counting.
The detector has the following data :
Area Detector = 119 cm2
Normal Cosmic Ray Flux= 0,019 particles/s cm²
Expected Measured Flux : 119 cm2 x 0,019 particles/s cm2 = 2,261 particles/s = 136 cpm
The geographical position of the detector is the following :
Latitude = 46° 04′ 01” N
Longitude =11° 09′ 18” E
Elevation = 398 m Mean Sea Level
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