A Muon Veto for the Ultra-Cold Neutron Asymmetry Experiment Vince Bagnulo LANL Symposium 2006 Outline ● UCNA Experiment ● Muon background ● Proposed Veto Design ● My Tasks ● Future Work

UCNA Experiment ● 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 Desired precision: Expected Signal Rate: 5-50 Hz Background rate: 3 Hz (predominantly cosmic ray muons)

Experimental Area B: UCNA Experiment From UCN source “prepolarizer” magnet polarizer magnet beta-spectrometer magnet UCN guide path beta detector

What are cosmic ray muons? ● Muons are a fundamental particle, very similar to electrons, but heavier (m  = 207 m e ) ● 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  cos 2 (  ) - flux  1 muon/ cm 2 / 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 10 6

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 cm 2 ) Cosmic Ray Muon ??? Compton Effect

Testing Method ● Add second trigger paddle and required coincidence to lower background ● Estimate double Compton scattering could occur to % 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