The Surface Rejection with Position Information Gensheng Wang California Institute of Technology CDMS Collaboration Meeting, UCBS Feb. 11, 2005

Overview Do we need to improve the beta rejection efficiency? How? Position information based beta rejection Summary

Surface Event Rejection with Timing Surface electron recoils could be low yield, but have fast timing Near neighbor low yield double scatterings as ejectrons minimum risetime rejects the surface recoils phonon delay rejects the surface recoils

Phonon Timing Distribution Ejectron at a big radius is most likely to fail flat surface rejection timing cuts

1 2 3 Geometry Parameter Because of phonon traveling distance, solid angle and side reflection, therefore But pfrac behaves in the central area of the detector

Separating Betas and Neutrons Phonon starttime Radius

Opposite Quadrant Phonon Energy Distribution A DC B shared phonon energy is expected to increase at big radius choose the radius cutoff value (between 2 and 3 cm) to allow pfrac behave in a reasonable fashion

A Handle of Discrimination R- phonon partition partition=(pa+pb- pc+pd)/pt for event in quadrant A Phonon partition is a symmetrical parameter between the phonon sensors, but pfrac is not Ejectrons can be discriminated in phonon partition and starttime two dimensional plane. RQ - no position correction neutrons (blue dots) and ejectrons (green crosses) R118 Z5

Red: ejectrons at the top of the detector Green: ejectrons at the bottom Circles: ejectrons in 3  nuclear recoil band Blue dots: neutrons keV ejectrons keV ejectrons phonon partition--pdelc

Ejectron at A Small Radius Phonon timing discrimination parameters are generally very good, only care should go to the exact centre area of the detector with phonon starttime and local quadrant phonon risetime. Luke phonons go away then are reflected back for surface events at phonon sensor side, but Luke phonons hit the phonon sensors directly for surface event at charge side. Phonon flux distribution as a function of time. Can we discriminate surface event at phonon side and surface event at charge electrode side? risetime ratio: (PAr50-PAr20)/(PAr80-PAr50) for an event in quadrant A Look at the data

Red: ejectrons at the top of the detector Green: ejectrons at the bottom Circles: ejectrons in 3  nuclear recoil band Blue dots: neutrons keV ejectrons keV ejectrons risetime ratio--pdelc

Using Run 119 Calibration Data All open Ba calibration data except _1252 Neutron calibration data cQin_119 & cChiSq_119 & cQThresh_119 & ~cVT gband_qi_arov119, yband_qi_arov119

Red: keV ejectrons at the top of the detector Black: keV ejectrons at the bottom Cyan: keV ejectrons not near doubles Magenta: keV ejectrons at the top of the detector Green: keV ejectrons at the bottom Yellow: keV ejectrons not near doubles Circles: ejectrons in 3  nuclear recoil band Blue dots: neutrons Surface Events Rejection at Small Radius

Surface Events Rejection at Big Radius Red: keV ejectrons at the top of the detector Black: keV ejectrons at the bottom Cyan: keV ejectrons not near doubles Magenta: keV ejectrons at the top of the detector Green: keV ejectrons at the bottom Yellow: keV ejectrons not near doubles Circles: ejectrons in 3  nuclear recoil band Blue dots: neutrons

Surface Event Cuts for Ge Detectors c119 – defined in risetime ratio-pdelc plane (R<2.5cm, transformation parameter) or phonon partition-pdelc plane (R  2.5 cm, transformation parameter). Defined c119 is R119 ebook 104. crt—defined as pminrtc > the mean of pminrtc of ejectrons in 3  NR and ptrtc > the mean of ptrtc of ejectrons in 3  NR, it’s a safety guard Events that pass both c119 and crt are selected as nuclear recoils.

T1Z1T1Z2T1Z3T1Z5T2Z3T2Z5 2  NR before  NR after  NR before  NR after Beta1 before Beta1 after Beta2 before Beta2 after Beta Rejection in Ba calibration Data ( keV)

The Leakage Event in T2Z3 ~cVT ~nearest double In 2  NR band pric=12.2 keV pdelc=14.5  s pminrtc=10.6  s ptrtc=17.0  s R=1.82 cm Small risetime ratio

T1Z1T1Z2T1Z3T1Z5T2Z3T2Z5 2  NR before  NR after  NR before  NR after Beta1 before Beta1 after Beta2 before Beta2 after Beta Rejection in Ba Calibration Data (5-10 keV)

The Neutron Selection Efficiency of c119 and crt Cuts Only

Summary Enhanced surface event rejection efficiency is achieved with radius parameter, phonon risetime ratio and phonon partition The original features of phonon parameters and position corrected phonon timing parameters are used together The optimizations of R and ejectron rejection efficiency could be done

The Outliers in the Open Ba Data

T2Z3

Reconstructed x and y Charge Inner and Outer Share Event (R118 BKGD) Z5, 40 ~ 80 keV, charge inner events and charge share events Conventional delay plotReconstructed x and y plot