A Muon Veto for the Ultra-Cold Neutron Asymmetry Experiment Vince Bagnulo LANL Symposium 2006...
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Transcript of A Muon Veto for the Ultra-Cold Neutron Asymmetry Experiment Vince Bagnulo LANL Symposium 2006...
A Muon Veto for the Ultra-Cold Neutron Asymmetry Experiment
Vince BagnuloLANL Symposium 2006
Outline●UCNA Experiment●Muon background●Proposed Veto Design●My Tasks●Future Work
UCNA Experiment
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● Ultra-cold neutrons totally reflect off specific surfaces
● Can be in stored in containers
● UCNA experiment measures the A-parameter of neutron beta
decay ( ) using trapped neutrons ● Can be used to check consistency of Standard Model
epen
Desired precision:
Expected Signal Rate: 5-50 Hz
Background rate: 3 Hz(predominantly cosmic ray muons)
Experimental Area B:UCNA Experiment
From UCNsource
“prepolarizer” magnet
polarizer magnet
beta-spectrometer magnet
UCNguide path
beta detector
What are cosmic ray muons?
● Muons are a fundamental particle, very similar to electrons, but heavier (m = 207 me)
● Cosmic rays are high- energy particles, mainly protons, from the sun and extra-solar sources
● Cosmic rays impinge on Earth's atmosphere, causing cascade of secondary particles – including muons
● At ground level, cosmic ray muons have the following parameters: - E 4 GeV
- angular distribution cos2() - flux 1 muon/ cm2/ min
Correcting for Muon Background● Three options for correcting background:
(1) shielding (2) background subtraction (3) active veto
● Veto detectors detect unwanted particles so they can be cancelled out
● Shielding is not an option: - Muon energy 4 GeV (on average) - Lose 2 MeV/cm in most materials - 20 m of concrete required - 24 million tonnes of concrete to shield Area B
● Background subtraction requires knowing background to 2% (which may not be possible)
● Must use active veto system
● Or 3.5 million elephants
3.5 x 106
Proposed Muon Veto
● Goal: Intercept cosmic ray muons that will impinge on primary detectors with efficiency of at least 90%
● Solution: Surround primary detectors with plastic scintillator detectors:
Cosmic Ray Muon
Muon Veto Paddle Beta spectrometer
Beta detector
Scintillator Detectors
● Scintillator re-emits energy deposited by ionizing radiation in the form of visible light.
● Visible light photons transmitted to photomultiplier tube (PMT) through use of total internal reflection
● A photomultiplier tube (PMT) attached to the plastic scintillator converts the light to an electronic signal and amplifies it
Charged particle
Scintillator PMT
Photons
Initial State (beginning of summer 2006)
● One prototype muon veto paddle constructed● Challenges: - veto paddles constructed out of salvaged
scintillator (effectiveness uncertain) - PMTs located in 30 gauss field
- scintillator paddles are large and unwieldy
10 feet
My Task
● Measure efficiency of prototype veto paddle as a function of position: - establish testing procedure
- help evaluate quality of scintillator● Help evaluate feasibility of plastic scintillator veto concept● Build production veto paddles
Developing a Testing Method
• Efficiency = detected muons/ incident muons
• Used cosmic ray muons to test efficiency
• Using one trigger paddle found efficiency 0%
• High singles rate due to gamma background:
Muon Veto Paddle
Trigger Paddle (200 cm2)Cosmic Ray Muon
???
ee
Compton Effect
Testing Method
● Add second trigger paddle and required coincidence to lower background
● Estimate double Compton scattering could occur to 0.1 - 1% of background gammas
● Two trigger paddle coincidence rate: 4.0 Hz
● Expected: 3.33 Hz
● Still not sufficient. Three trigger paddles might be necessary
Photon
Electron
Muon Veto Paddle
Cosmic Ray Muon
Trigger Paddle
Testing Results (two trigger paddles)
● Measured efficiency drops to 50% at far end of veto paddle
● A glue joint is broken
● Repairs problematic start construction of first production paddle
Broken glue joint
Conclusion
● Currently, performance of prototype veto paddle is not sufficient
● A new paddle is being constructed and will be tested
● If satisfactory, remaining paddles will be constructed
● If not, alternative methods of carrying out veto will be investigated