Precision Measurement Of Nuclear Recoil Ionization Yields For Low Mass Wimp Searches

Precision Measurement Of Nuclear Recoil Ionization Yields For Low Mass Wimp Searches Tarek Saab University of Florida Tali Figueroa Northwestern University Calibration of Low Energy Particle Detectors Workshop Chicago, 2015

The SuperCDMS Collaboration in 2015

- [ ] Si HV Ge HV CDMSlite (2013) SuperCDMS S NOLAB 7 Be Neutrinos DAMIC (2012) Ge izip SuperCDMS LT (2014) COHERENT NEUTRIN O SCATTERING 8 B Neutrinos CoGeNT (2012) CDMS Si (2013) CRESST DAMA COHERENT NEU TRI NO SCATTERING [ / ] SIMPLE (2012) EDELWEISS (2011) CRESST (2014) DEAP3600 XMASS Atmospheric and DSNB Neutrinos COUPP (2012) ZEPLIN-III (2012) CDMS II Ge (2009) Xenon100 (2012) LUX (2013) LUX 300day SuperCDMS SNOLAB Xenon1T LZ COHERENT NEUTRINO SCATTERING - [ ] SuperCDMS and HV Detectors Sensitivity The WIMP Search Landscape

The main theme Now that we re in the low-mass wimp search era, how can we push the measurement of nuclear recoil ionization yield down to as as low an energy as possible using cryogenic crystal (Ge/Si) detector technology? Integrated Rate @evtêkgêyeard 200 100 50 20 10 5 2 1 Total Rate for different thresholds in Ge, s = 1. 10-42 cm 2 Same Target (Ge) Different WIMP Masses 3 4 5 6 7 8 9 10 GeV 0 2 4 6 8 10 Experimental Threshold @kevd

Defining Ionization Yield in Crystal Detectors For a given interaction with recoil energy E recoil all of the recoil's energy goes into the various "prompt" channels, e.g. in Ge: E recoil = E phonon Additional, delayed energy will go into the phonon channel due to 1. The e-h recombining at the electrodes and 2. Luke phonon emission from drifting charges. We define the total phonon energy as: E phon. = E phonon! is the average amount of energy it takes to create an e-h pair. n eh is the number of e-h pairs that reach the electrodes, and ev is the voltage across the detector times the electron's charge. Since E ioniz. = n eh!, we get: prompt + E ioniz. prompt + n eh E eh + n eh ev E phon. = E recoil + E ioniz. ev E eh

Ways to measure y and σy For any determination of y, we need to measure two quantities independently: Erecoil and Eioniz. y = E ioniz. E recoil! y = s 2 rec E 2 rec + 2 ion E 2 ion We need to be able to measure both quantities with (similar) good resolution

The SuperCDMS HV Detector Operation Phonon-based charge amplification Phonon energy = Erecoil + ELuke = Erecoil + neh ev h+ Prompt phonons E field e-

The SuperCDMS HV Detector Operation Phonon-based charge amplification Phonon energy = Erecoil + ELuke = Erecoil + neh ev h+ Luke phonons Prompt phonons E field e- Luke phonons

Ways to measure y and σy using a HV detector With a HV detector we measure Ephon. only, which effectively gives us Eioniz. with a very good resolution. By measuring the recoiling particle s scattering angle we can determine kinematically the recoil energy Erecoil with a very good resolution. y = E eh ev Ephon. 1 E recoil! y = q Erec 2 ph 2 + E2 ph rec 2 ev E eh E 2 rec Look at that nice big number in the denominator

Taming the low energy divergence Comparing the ±1σ yield uncertainty bands for a generic" yield measurement with σrecoil = σioniz vs a Voltage-Assisted Calorimetric measurement (HV detector) Assuming: σphon = 50 ev, σrecoil δφ=1, V=100V. Expected Ionization Yield Resolution 0.4 0.3 Ionization Yield 0.2 0.1 0.0 0.01 0.05 0.10 0.50 1 5 10 Nuclear Recoil [kev] The bands represent ± 1σ

Yield resolution vs V at σphon = 100 ev The x-axis is the operating bias voltage, and the different colors are various phonon resolutions 1000 500 Expected Ionization Yield Resolution Phonon sensor 1σ resolution 100 ev This point corresponds to the orange bands on the previous page at 100 ev recoil σ y /y [%] 100 50 50 ev 20 ev 10 ev 10 5 1 15% 7% 5 10 50 100 500 1000 Bias Voltage [V] At this point the σrecoil δφ=1 begins to dominate

0 th round calculations The basic simulation

Simulating the ideal" experiment Select n beam energy 30 kev monochromatic Select bias voltage V Calculate Erecoil for a set of scattering angles φ Generate recoil energies Erecoil with σrecoil given by δφ=±1 Calculate #e-h pairs using Lindhard yield and Fano factor statistics Calculate Ephon from Erecoil + Luke phonons + σphonon Calculate yield from Ephonon, Erecoil, and V: y = E eh ev Ephon. E recoil 1 12

The Baseline" Simulation: Ge target Parameters: Ek(n)=30 kev, σphon=50 ev, σrecoil ± 1, V=100V, F=0.13 No fit was done. The black box is the mean of 0.4 the points, the bar height is the variance. Expected Ionization Yield Resolution 0.3 Ionization Yield 0.2 0.1 0.0 0.01 0.05 0.10 0.50 1 Nuclear Recoil [kev]

The Baseline" Simulation: Ge target Parameters: Ek(n)=30 kev, σphon=50 ev, σrecoil ± 1, V=100V, F=0.13 This is how the data would fit in with current knowledge 0.4 Expected Ionization Yield Resolution in Ge Ionization Yield 0.3 0.2 0.1 0.0 0.01 0.10 1 10 100 1000 Nuclear Recoil [kev]

The Baseline" Simulation: Si target Parameters: Ek(n)=30 kev, σphon=50 ev, σrecoil ± 1, V=100V, F=0.13 This is how the data would fit in with current knowledge Ionization Yield 0.4 0.3 0.2 0.1 Expected Ionization Yield Resolution in Si This work Dougerthy (1992) Gerbier et. al. (1990) Sattler(1965) 0.0 0.01 0.10 1 10 100 1000 Nuclear Recoil [kev]

A Still Not-Totally-Crazy Simulation: Ge target Parameters: Ek(n)=30 kev, σphon=10 ev, σrecoil ± 1, V=100V, F=0.13 Expected Ionization Yield Resolution 0.4 0.3 Ionization Yield 0.2 0.1 Now we re seeing individual electrons/holes 0.0 0.01 0.05 0.10 0.50 1 Nuclear Recoil [kev]

1 st round calculations Performing the measurement

Multiple interactions in a detector There are two types of multiple interactions to consider 1. One neutron undergoing multiple scatters 2. Different neutrons from the same bunch undergoing coincident scatters Can t really identify one from the other

One neutron scattering multiple times in a detector 30 kev Neutron m.f.p. in Ge: 23 mm probability of a neutron interacting in a 4mm thick detector is 18%. i.e. probability of NOT interacting at all is 82% Si: 118 mm probability of a neutron interacting in a 4mm thick detector is 3%. i.e. probability of NOT interacting at all is 97%

Probability of n simultaneous interactions vs navg These are the events we want p 1.0 0.8 0.6 0.4 0.2 p(0 interactions) p(1 interactions) p(>1 interactions) 0.15 0.10 0.05 p(>1 interactions) p(1 or more interactions) We want this to be as small as possible 0 1 2 3 4 5 navg 0.5 1.0 1.5 2.0 navg navg = average number

Minimizing multiple interactions To achieve a multiple interaction to single interaction ratio of < 3% requires that navg=0.4 At navg=0.4, the probability of a single interaction from a neutron bunch = 6% Limiting the maximum interaction rate in the detector to 100 Hz (to avoid pileup, assuming 1 ms decay time) gives a desired bunch frequency of 1/600μs. The combination of bunches arriving every 600μs with navg=0.4 is feasible with the TUNL facility

2 nd round calculations Some unwanted" effects

Neutron interactions prior to the detector Assuming a perfectly monochromatic neutron beam with no angular spread is incident on the experiment Look at effect of neutrons interacting in the material surrounding the detector on the purity" of the incident beam

Shoving* Geant at the problem *The simulation was too basic to use the phrase Throwing Geant at the problem with a straight face. Simple Geant MC of 30 kev neutrons incident on: 4mm thick Ge detector surrounded by 1mm thick Cu housing, enclosed in 1cm thick Al ADR

Recoil Energy vs Scattering Angle Total Energy in Detector [kev] Simulated Detection Energy Depositions 3500 Legend Multiple scatters in detector 3000 Single scatters in detector These are the events we want Scatter in detector and Al cryostat Scatter in detector and Cu holder 2500 2000 1500 1000 500 0 0 50 100 Scattering Angle θ [ ] 150

Can use timing information The neutrons are slow enough that the timing between the neutron bunch and its detection in a PMT depends on the recoil energy in the ZIP. Event delay time distribution A timing cut can clean up the data quite a bit Number per 5 ns bin 1000 100 10 Legend All scatters in detector and Al/Cu Multiple scatters detector only Single scatters in detector only In reality things are much better that this histogram indicates, since this includes all events at all energies Looking in specific energy bins the contamination is 1%. 1 0 100 200 300 400 Δt [ns]

How monochromatic is the n beam? Energy loss of the proton beam due to scattering in the LiF target prior to the production of the neutrons will introduce an energy spread in the neutron beam For 1.88-1.92 MeV protons incident on a 75 nm thick LiF target average proton energy loss due to scattering is ~2.5 kev, up to 5 kev For 1.88-1.92 MeV protons incident on a 500 nm thick LiF target average proton energy loss due to scattering is ~8.0 kev, up to 15 kev Results in spread of neutron energies

Neutron energy spread due to LiF thickness 10 5 Protons incident on a 75 nm target E n =32keV E n =34keV E n =52keV E n =116keV δe n =1keV δe n =2keV δe n =3keV δe n =2keV 10 4 1000 Neutron production efficiency 100 10 Enominal= 60 kev Enominal= 120 kev 1 0 20 40 60 80 100 120 140 En [kev]

Recovering from the neutron energy spread Timing measurement of the neutron interactions can help recover the initial (pre-scattering) neutron energy At 30 kev, a time of flight difference of 5ns allows us to identify δen = 0.25 kev At 120 kev, a time of flight difference of 5ns allows us to identify δen = 2 kev Choosing the optimal tradeoff between neutron production rate and energy resolution will be done in upcoming Geant simulations

Final thoughts almost the end

Calibration of SuperCDMS detectors with a photoneutron source Lauren Hsu will give a presentation about ongoing efforts to measure yield in the SuperCDMS detectors using photoneutron sources. Thu, Session III

Conclusions Measuring the ionization yield below 1 kev nuclear recoil requires measuring the ionization produced to within a few electron-hole pairs Phonon-based charged amplification using our HV detectors will allow us to attain this charge resolution. A dedicated HV detector running at a neutron beam facility will enable the first measurements of yield in the energy range of 100 ev 1 kev, in Si and Ge. Verifying, or measuring any deviations from, Lindhard behavior in this energy range and at the operating temperature and E field is essential for interpreting future low-mass WIMP data.