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Initial Tests Of The Pion Beta Decay Detector



David Lawrence

A Dissertation Presented in Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy



The pion beta decay experiment being performed at the Paul Scherrer Institute in Villigen, Switzerland intends to measure the decay rate to a precision of <0.5% uncertainty. The current measurements have a 3.8% uncertainty. A precise determination of the decay rate will provide stringent tests of the Conserved Vector Current hypothesis (CVC), radiative corrections, and Cabibbo-Kobyashi-Maskawa quark mixing matrix (CKM) unitarity. CKM unitarity is contingent upon a three quark model. A clear violation of CKM unitarity could signal the existence of additional quark generations.

The pibeta detector consists of several pieces. These include a large segmented calorimeter constructed of pure CsI, a thin plastic scintillator array, an active target and degrader, and set of cylindrical multi-wire proportional chambers. Special attention is paid to the thin plastic scintillator array.

Cosmic rays were used to measure the attenuation length and signal velocity in three of the plastic scintillator staves. The results were consistent with the nominal attenuation length and signal velocity values reported for the material from which the staves were constructed.

A measurement of the branching ratio was performed in 1997. The value for the branching ratio was extracted using a combination of simulated and measured spectra. The valued obtained is which is consistent with the currently accepted value of [PDG 96] for the branching ratio.


I would like to thank my father and stepmother

Roger Lawrence and Joyce Webb-Lawrence

for all their help and support

I would like to thank my mother and stepfather Sue and Ralph Gragg

for all their love and support

I would like to thank Professor Barry Ritchie

for his help and guidance through all aspects of this work

I would like to thank

Professor Dinko Pocanic, Dr. Emil Frlez, and Dr. Stefan Ritt

for their invaluable contributions to my education while performing this work

I would like to thank Professor William Kaufmann

for several very useful theory discussions

I would like to thank my committee

Professors Ricardo Alarcon, Raghunath Acharya, Ralph Chamberlin, and Richard Jacob

for their part in my educational process

Mostly, I'd like to thank my wife

Heather Hill

for her deep love and support throughout my entire graduate career

and our daughter

Amelia Lawrence

who's completely contagious smile will forever amaze me


Table Page1.1: The decay modes of 4

1.2: Values for the nuclear mismatch correction (in percent) for superallowed beta decays in several nuclei as calculated by various sources 9

4.1 : General properties of BC-400 organic scintillator 73

4.2 : Fit results for inverse attenuation lengths from PV tomography data 81

5.1 : Channel groupings used for common mode noise rejection 88

5.2 : Values for 104

5.3 : Values used to determine 108


Figure Page1.1: Experimental setup from [Cza 93] 11

1.2 : The experimental setup from [Bri 92,Bri 94] 13

1.3 : Positron energy spectrum for early times (<30 ns) from [Bri 94] 14

1.4 : The experimental setup from [Dep 68] 15

1.5 : The experimental setup from [McF 85] 16

3.1 : Overview of the PSI accelerator facility taken from the 1994 user's manual 38

3.2 : Schematic of a Cockroft-Walton type DC accelerator like that used at PSI as a pre-injector 39

3.3 : Injector II 41

3.4 : Ring Accelerator 42

3.5 : Schematic of the PSI beamlines 44

3.6 : Production target at PSI 45

3.7 : Diagram of the pion beta decay detector [Brö 96] 48

3.8 : The granular target constructed in 1994 and used in the 1994 beam time 49

3.9 : CsI crystals, half-length plastic veto, target, degrader, target veto from 1996 GEANT simulation 50

3.10 : Arrangement of the detectors used in the 1997 beam time 52

3.11 : The 1996 and 1997 experimental apparatus 53

3.12 : Target and degrader 55

3.13 : Plot made from one of the plastic veto half-staves during the 1997 beam time 57

3.14 : Analog leg of the data acquisition electronics 61

3.15 : Trigger leg of the data acquisition electronics 62

3.16 : Inputs and ouputs of the LRS 2365 trigger module at the heart of the trigger electronics 64

3.17 : Trigger timing for several signals created and used in the electronic trigger logic 67

4.1 : End view of a plastic veto scintillator stave 71

4.2 : Side view of a plastic veto scintillator stave 72

4.3 : Light output vs. wavelength for BC-400 organic scintillator 74

4.4 : Tomography setup at PSI with PV staves inside 78

4.5 : A PV stave of length L with a particle passing through at the position indicated 79

4.6 : Scatter plots used to determine the effective attenuation length of a PV stave 80

4.7 : Scatter plots of distance along stave (mm) vs. signal time(0.1 x ns) 83

5.1 : Pedestal histograms generated from raw ADC values 87

5.2 : CsI sum spectra with high energy threshold 90

5.3 : CsI sum spectra with low energy threshold 91

5.4 : Timing spectrum for events in the DPG 92

5.5 : Energy spectrum for the target for events in the DPG and DPG' 93

5.6 : Timing spectrum using the prompt trigger 94

5.7 : Timing spectrum for prompt events in the DPG 95

Figure Page5.8 : CsI sum spectrum from 1996 data made by illuminating the CsI array with a 70 MeV positron beam 96

5.9 : Experimental Michel spectrum from the CsI array 97

5.10 : Spectra used to determine Rts 99

5.11 : Spectra used to determine Rlh 100

5.12: Timing spectra used to determine gate delay 105

5.13: Low energy end of the experimental Michel spectrum with theoretical curve 106

Dean, Graduate College

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