Other generator/muonprophet

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MuonProphet

Motivation

MuonProphet [DocDB 4153, DocDB 4441] is designed to address the simulation of muon which will be a major background source of Daya Bay neutrino experiment. Spallation neutrons and cosmogenic background, namely 9Li, 8He etc., are supposed to give the biggest systematic uncertainty.

The vast majority of muons are very easy to identify due to its distinguishable characteristic in reality. Usually its long trajectory in water pool or AD will leave a huge amount of light and different time pattern rather than a point source.

The simulation of muon in Geant4 is quite time-consuming. The hugh amount of optical photons’ propargation in detector, usually over a few million, can bring any computer to its knee. One CPU has to spend 20-30 minutes for a muon track sometimes. The real muon rate requires to simulate is a few hundred to a thousand per second.

In the end people realized that they only need to know whether a muon has passed the detector and tagged, while not really care too much about how light are generated and distributed in water pool and AD.

Beside that it is technically impossible to finish all these muon simulation, the physics model of radioative isotope’s generation in Geant4 is not very reliable. Photon nuclear process triggered by virturl or real photon, pion- nucleus interaction, nucleon-nucleus interaction, etc. are all possible be responsible to spallation background generation. They are poorly described in Genat4. Tuning the generation rate of some background is very difficult, since they are usually very low, then it is very inefficient to do MC study.

Based on these consideration MuonProphet is designed so that the tiresome optical photon simulation can be skipped and the generation of spallation background can be fully controled and fully simulated by Geant4.

Generation Mechanism

Firstly it starts from a muon track with initial vertex and momentum. The intersections of the muon track with each sub-detectors’ surface and track lengths in each segment are calculated. Low energy muon could stop in detector according to a calculation based on an average dE/dx. According to its track length in water and whether it crossed RPC and user configuration it will determine whether this track is going to be triggered. Spallation neutron and cosmogenic background generation rate is usually a function of muon’s energy, track length and material density. According to a few empirical formulas from early test beam and neutrino experiments, spallation neutron and/or radioactive isotopes are generated around the muon track. Because water is not sensitive to radioactive isotopes and their initial momentum is very low, they are only generated in AD. Muon is always tagged as “don’t need simulation” by a trick in Geant4. However neutron and radioactive isotope are left for full Geant4 simulation.

Code Organisation

Besides the big structure determined by the motivation most parts of the codes are loosely bound together. Under MuonProphet/src/functions, all generation probabity functions, vertex and energy distribution functions are included. They can easily be modified and replaced. Under MuonProphet/src/components, MpGeometry.cc is dedicated to geometry related calculation; MpTrigger.cc is for trigger prediction; MpNeutron.cc and MpSpallation.cc handle the production of neutron and other isotopes respectively. All of them are controlled by MuonProphet::mutate like a usual gentool. It will make use of other radioactive background generators, so no need for extra code development.

Configuration

Here one example is given for 9Li or 8He background configuration. It will create a gentool - prophet. This tool should be attached after muon GtPositionerTool, GtTimeratorTool and GtTransformTool like demonstrated in MuonProphet/python/MuonProphet/FastMuon.py . According the formulas in [DocDB 4153, DocDB 4441] a set of four parameters including a gentool for an isotope background, yield, the energy where the yield is measured and lifetime must supplied. Following is a snippet of python code from FastMuon.py showing how it is configured.

# - muonprophet
prophet=MuonProphet()
prophet.Site= ``DayaBay''
# - spallation background
## - The tool to generate 9Li or 8He background
## - According to the formula refered in [DocDB 4153, DocDB 4441]
## - every isotope need a set of four parameters.
prophet.GenTools= [ ``Li9He8Decayerator/Li9He8'' ]
## - There is a measurement of yield 2.2e-7 cm2/g for 260 GeV muon,
## - then we can extrapolate the yield to other energy point.
prophet.GenYields= [ 2.2e-7 *units.cm2/units.g ]
prophet.GenYieldMeasuredAt= [ 260*units.GeV]
## - The lifetime of them is set to 0.002 second
prophet.GenLifetimes= [ 0.002*units.s]
# - trigger related configuration
## - Any muon track with a track length in water above 20 cm will be tagged as triggered.
prophet.TrkLengthInWaterThres= 20*units.cm
## - We can also assign a trigger efficiency even it passed above track length cut.
prophet.WaterPoolTriggerEff = 0.9999

Output

Geant4 will skip the muon simulation and do full simulation for neutron and other isotopes. The rest of the simulation chain in Fifteen is set up to be able to respond that correctly. Electronic simulation will only simulate the hits from spallation background and only pass a empty ElecHeader for the muon to the next simulation stage. If muon is tagged triggered, then trigger simulation will pop out a trigger header for the muon, otherwise, it will be dropped there like the real system.

In the final output of readout stream, user should expect the following situations: a) Only muon is triggered. There will be an empty ReadoutHeader for muon. User can trace back to the original GenHeader to confirm the situaion. b) Only spallation background is triggered. c) Both muon and background induced by this muon are triggered. There will be a empty ReadoutHeader for muon and another one with hits for the background. d) No trigger.

In reality if there is something very close to the muon in time, their hits will overlap and their hits are not distinguishable. For example, some fast background following muon won’t be triggered separately. User should do the background trigger efficiency calculation based on the understanding of the real Daya Bay electronics.

Trigger Bits

Although the output got from MuonProphet simulation is empty, i.e. no hit, but the trigger information is set according to the fast simulation result. According to the geometry input it could have RPC and waterpool trigger.

Quick Start

There is one example already installed with nuwa. After you get into nuwa environment, you can start with

> nuwa.py -n50 -o fifteen.root -m "MuonProphet.FullChain" > log

It will invoke the FastMuon.py.