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

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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

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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