The Pion Beta Decay Experiment and A Remeasurement of the Panofsky Ratio

4.3.2 On-line Results

After equalizing the gains of the 44 CsI-modules the energy resolution was determined using a 70 MeV positron beam. Two plane MWPCs were used to accept only positrons of proper momentum by excluding beam divergencies of more than 2%. Sum histograms were generated for the innermost 18 crystals by adding all CsI modules whenever the crystal of interest got the main fraction of the shower energy. The differences in the obtained energy resolutions reflect the measured number of photoelectrons and non-uniformities. The average contribution from the noise of approximately 0.1 MeV is negligible.

Crystal Channel	Crystal Name	N_ph	Uniformity [%/cm]	FWHM in %	s [MeV]	calculated s
C11	S161	78	0.055	5.4	1.6	1.4
C12	S064	78	-0.035	5.5	1.6	1.4
C13	S028	71	0.660	5.9	1.8	2.4
C14	S035	81	-0.175	5.2	1.5	1.4
C15	S068	50	0.045	6.9	2.1	1.7
C20	S115	66	0.065	5.3	1.6	1.5
C21	S112	69	0.355	5.8	1.7	1.8
C22	S029	79	0.075	5.7	1.7	1.4
C23	S002	116	0.370	5.3	1.6	1.6
C24	S021	79	-0.145	5.3	1.6	1.4
C25	S114	81	0.320	6.3	1.9	1.6
C26	S117	70	0.190	6.0	1.8	1.5
C29	S165	89	0.585	7.5	2.2	2.1
C30	S062	83	0.055	6.3	1.9	1.3
C31	S031	51	0.050	5.8	1.7	1.7
C32	S061	93	0.135	5.9	1.8	1.3
C33	S162	70	-0.015	5.4	1.6	1.4
C37	S130	64	0.025	5.9	1.8	1.5

Table 4-2 Summary of the obtained energy resolutions of the inner CsI modules during the 1997 beam period obtained with 70 MeV positrons directly impinging the array. The figures are obtained by summing over the whole array, when the crystal of interest achieves the main fraction of an electromagnetic shower.

The average energy resolution (FWHM) of the inner part of the array for 70 MeV positrons was determined to be 4.2 MeV. Most of that amount can be directly attributed to the optical properties of the individual modules. A major contribution comes from shower spread over several modules with different uniformities and intercalibration uncertainties. Minor contributions are due to electronic noise.