A muon[
High Power Laser Science and Engineering, Volume. 5, Issue 3, 03000e16(2017)
A new method on diagnostics of muons produced by a short pulse laser
Muons produced by a short pulse laser can serve as a new type of muon source having potential advantages of high intensity, small source emittance, short pulse duration and low cost. To validate it in experiments, a suitable muon diagnostics system is needed since high muon flux generated by a short pulse laser shot is always accompanied by high radiation background, which is quite different from cases in general muon researches. A detection system is proposed to distinguish muon signals from radiation background by measuring the muon lifetime. It is based on the scintillator detector with water and lead shields, in which water is used to adjust energies of muons stopped in the scintillator and lead to against radiation background. A Geant4 simulation on the performance of the detection system shows that efficiency up to 52% could be arrived for low-energy muons around 200 MeV and this efficiency decreases to 14% for high-energy muons above 1000 MeV. The simulation also shows that the muon lifetime can be derived properly by measuring attenuation of the scintilla light of electrons from muon decays inside the scintillator detector.
1 Introduction
A muon[
In general, the muon is produced as a secondary cosmic ray from the
Another possible source of a muon with both signs is the Bethe–Heitler lepton pair production process[
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To validate the dimuon production by a short pulse laser, a suitable muon diagnostics is needed. Although there were lots of muon detectors since its discovery[
Our idea is to diagnose the muon lifetime to distinguish the muon signal. The produced muons could be slowed down to stop inside the detector. Then generally after an average lifetime
The lifetime measurement would give an explicit evidence for muon production by a short pulse laser. Furthermore, because muons decay later than the laser ‘shot time’ by hundreds of ns, the radiation background would attenuate dramatically. To stop muons of different energy in the scintillator, water with different radiation lengths could be employed.
In this paper, we show results of the Geant4 simulation of muon diagnostics process described above. In Section
2 Simulation setup of the diagnostic system
The dimuons can be produced by a short pulse laser as shown in Figure
After that an electron with energy range of 0–51 MeV is produced which have a maximum stopping range around
A layer of 5 cm lead is introduced to shield the radiation background in the experiment. Since the detection time is later than the laser shot time by several hundreds of ns, only very few secondary particles such as high-energy photons and neutrons have effects on muon detection. It is located behind the water and close to the scintillator. Muon scattering with the lead shield would enlarge the emittance of muon source resulting in lowering the detection efficiency.
Considering the finite volume of the scintillator, only muons with proper energy (less than 100 MeV) would stop inside the detector and decay consequently. In the case of higher-energy muons, water is employed to decelerate the muons by the minimum ionization process [
The detection system was simulated by using the Monte Carlo code Geant4. Geant4 is a toolkit for the simulation of the passage of particles through matter[
3 Detection efficiency
To simulate ‘successful catch’ muon events by Geant4, a muon source is placed on the axis of the detection system with a flat energy distribution from 200 to 1000 MeV. The laser wakefield accelerated electron beam typically has a radius with
The physical processes of muons including decay, nuclear reaction, multi-scattering, ionization, bremsstrahlung radiation and electron pair production are considered in the simulation. Information of electrons decayed from muons inside the scintillator is recorded to get a ‘successful catch’ event.
To ‘successfully catch’ muons of different energy, the water length is changed from 60 to 380 cm with a 40 cm step. The cross-section of water in the simulation is
Totally
For finite scintillator volume, the individual peak in Figure
4 The muon lifetime diagnostics
The muon lifetime could be derived by measuring the scintilla light produced by the electrons decayed from muons stopped inside the scintillator detector. When abundant muons are stopped inside the detector, electrons are generated accordingly and deposited rapidly. Thus, in the large sample limit the attenuation of the scintilla light followed an exponential distribution with an average time of the muon lifetime.
In general, the scintilla light yield is proportional to the deposited energy of the electrons but with a coefficient depending on the parameter of definite scintillator. For generalization, only the deposited energy of electrons was considered in this paper.
In the simulation discussed in Section
It is worth mentioning that the discussion above was obtained in a large sample limit assumption. For a small quantity of muons, since the electrons had an energy range from 0 to 51 MeV, the fluctuation in the tail of the distribution would be enhanced. Furthermore, in cases when only a few muons generated, the detection system can be run at muon counting mode, in which muon events could be identified as isolated peaks in the equipment, such as an oscillograph. The lifetime of muons can still be derived with the maximum likelihood fit on those isolated peaks.
5 Summary
Muons produced by short pulse lasers can serve as a new type of muon source that have potential advantages of high intensity, small source emittance, short pulse duration and low cost. The first step before we can apply it to fundamental problems of elementary physics such as neutrino physics, lepton physics or be used in
The detection system consisted of three parts: water, lead and scintillator. The scintillator detector is used to stop muons and catch the electrons from muon decays. Water and lead are used to decelerate the muons by ionization processes so that the muons would finally stop inside the scintillator. In addition, they also serve as the shielding against strong radiation background.
A Geant4 simulation of the whole detection system was performed. The ‘successful catch’ muon events were recorded to determine the detection efficiency of the system. It was shown that an efficiency up to 52% could be achieved for low-energy muons around 200 MeV but it decreases to 14% for high-energy muons up to 1000 MeV. The reason is that for high-energy muons, a longer water tank is needed to stop it which also increases the emittance of muon beams. By recording the deposited energy inside the scintillator detector, the muon lifetime could be derived from attenuation of the scintilla light of electrons from muon decays. It is also shown that the detection system can distinguish the muon signal from a strong radiation background such as high-energy photons. The detection system can be easily calibrated by the cosmic ray muons. After that the yield and energy spectrum of muons generated by a short pulse laser would be obtained straightforwardly in experiment.
The detection system can also be run at a counting mode in case of low muon flux. Considering the cross-section of the scintillator is
The detection system can be taken as a basic method to measure muons produced in laser experiments. For example, with the development of ion acceleration by a short pulse laser, muons generated by proton beams with energy exceeding muon generation threshold seem to be quite possible in the near future.
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Feng Zhang, Boyuan Li, Lianqiang Shan, Bo Zhang, Wei Hong, Yuqiu Gu. A new method on diagnostics of muons produced by a short pulse laser[J]. High Power Laser Science and Engineering, 2017, 5(3): 03000e16
Category: Research Articles
Received: Nov. 16, 2016
Accepted: May. 5, 2017
Published Online: Nov. 21, 2018
The Author Email: Yuqiu Gu (yqgu@caep.ac.cn)