The LZ Photoneutron Calibration Source

Andreas Biekert UC Berkeley

APS April Meeting April 15, 2019

on behalf of the LZ Collaboration

1

The LZ Collaboration

2

@ UC Davis, Jan. 2019

37 institutions; 250 scientists, engineers, and technicians

The LZ detector is being built at SURF in Lead, SD

The LZ Detector

3

High purity water

Gd-loaded liquid scintillator

Instrumented Xe skin region

LXe Time Projection Chamber 7 tons active Xe 5.6 ton fiducial volume

Veto system

Liquid scintillator filled plug

LZ TDR: arXiv:1703.09144

How LZ Works !! Energy deposited by particle

interactions "! S1 light: prompt scintillation "! S2 light: electroluminescence from

drifted electrons

!! 3D position reconstruction

!! Discrimination between electron recoils (ERs) and nuclear recoils (NRs)

!! Possibility of low energy S2 only analysis "! complicated by lack of discrimination

4

S1

S2

time

S2

S1

drift time

0 20 40 60 80 100Nuclear recoil energy [keV]

10�11

10�10

10�9

10�8

10�7

10�6

10�5

10�4

10�3

Rat

e[c

ount

s/kg

/day

/keV

]

8B

Atm

hep

DSN

Det. + Sur. + Env. Total

1 10 100Nuclear recoil energy [keV]

3!10

2!10

1!10

1

Effic

ienc

y

NR efficiency

Low Energy Nuclear Recoil Backgrounds

5

WIMP search criteria: S1 > 3 phd S2 > 5 e- S1 < 80 phd

Single scatter NR events in LZ fiducial volume*

Simulated NR detection efficiency, extrapolated by Lindhard theory below 1.1 keV**

LZ sensitivity paper: arXiv:1802.06039

* after outer detector and skin vetoes applied, before efficiency and S1 selection cuts

** multiple low-energy NR calibrations planned; see TDR

Photoneutron calibration

Gamma Source Neutron Energy [keV] Xenon Recoil Endpoint [keV]

88Y 153 4.6

205Bi 88.5 2.7

206Bi 47.5 1.4

124Sb 22.5 0.7

Photoneutrons

6

Overview: G. F. Knoll. Radiation Detection and Measurement. (2000).

Proposed as DM calibration: J. I. Collar. PRL,110(21), 2013.

•! Match gamma energy to Q for low energy neutrons

•! First order: monoenergetic neutrons from monoenergetic gammas

•! Process has fairly small cross section

This talk

Future work

A Photoneutron Calibration Source

7

stainless steel guide tube

photoneutron source assembly

LZ TPC

20 cm

20 cm

4 cm Tungsten cap

Tungsten shield block

Tungsten source capsule

BeO / 88Y source

What Could Go Wrong?

!! Radiation from YBe source: 1.! 3.7 MBq 88Y MeV-scale gammas 2.! 270 neutrons per second 3.! MeV-scale gammas from neutron capture

!! Key questions about event pileup: "! Will the total event rate overwhelm matching of S1s and S2s from photoneutron events? "! Will there be a well-defined endpoint in the photoneutron single scattering spectrum? "! Will we be able to distinguish signals from backgrounds such as delayed electron noise?

e.g. P. Sorensen and K. Kamdin. JINST, 13 P02032, 2018.

8

A Jurassic Park reference

Current Simulation Approach

!! Full model of LZ in a GEANT4-based simulation

"! 88Y gammas directly from decay in GEANT4

"! Photoneutrons from virtual gamma vectors #! Sample locations and momentum-

energy relation more accurately

"! First-order approximation from photoneutron cross section to scale datasets together

9

Photoneutron source

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Deposited Energy [keV]

4!10

3!10

2!10

1!10

1

Even

t Rat

e [C

ts /

keV

/ sec

]

Photoneutron NR

Y88 Gamma ER

Photoneutron ER

LZ Preliminary

The Total Event Rate !! No cuts on events after post-

processing

!! 3.7 MBq 88Y decay gammas main source of event rate

!! Gamma rate from neutron capture coupled to photoneutron rate (1st bin)

!! Source events towards top of TPC, so less time between S1 and S2 "! Define small fiducial region towards the top

of the detector as signal region

10

Total NR rate*: 1 Cts / sec Total ER rate: 206 Cts / sec

ER from 88Y decays: 179 Cts / sec

*Non-thermal neutrons

The Signal Region

11

80 events / day (S1+S2)

!! Signal region looks great!

!! S2 only (no S1 detected) events shown, but not the focus of this talk

0 10 20 30 40 50 60S1 [phd]

2.6

2.8

3

3.2

3.4

3.6

3.8

4

4.2

4.4

(S2

[phd

])10

log

LZ Preliminary

LZ Preliminary

Calibrating Expected Signals !! What 24 hours of calibration data could

look like

!! Radiogenic detector backgrounds negligible in fiducial region (<< 1 Cts / day)

!! Nice overlap with coherent neutrino scattering signal region, mainly from 8B solar neutrinos

12

8B + hep

40 GeV WIMP

Photoneutron NRs

ERs from source

Summary !! Incorporate full optical response, electronics

response, and analysis framework into simulation analysis "! e.g. “LZ Mock Data Challenge 3” happening soon

!! Incorporate more sophisticated event overlap and delayed electron noise in simulations

!! Consider physics simulation accuracy e.g. A. E. Robinson. PRC, 89(3), March 2014.

!! We will have a physical source later this year 13

We’re trying to avoid this

Special Thanks !! Lawrence Berkeley National Lab !! LZ Collaboration

"! Specifically: Junsong Lin, Quentin Riffard, Peter Sorensen, Rick Gaitskell, Dan McKinsey

!! NSF GRFP Program for support

14

Backup Slides

15

Outgoing Neutron Spectrum

16

!! 180 photoneutrons per second leave the tungsten shield block

!! Largest z-position of each event (single and multiple scatters)

!! Use mean z for back of the envelope Poisson calculation of event overlap fraction

!! Mean z: -160 mm !! Mean drift time 94 µs (assuming uniform

drift field)

Mean z of Events

17

1600! 1400! 1200! 1000! 800! 600! 400! 200! 0Distance to liquid level [mm]

6!10

5!10

4!10

3!10

2!10

1!10

1

Even

t Rat

e[C

ts /

mm

/ se

c]

Photoneutron NR

Y88 Gamma ER

Photoneutron ERLZ Preliminary

Fiducial Volume

18

0 100 200 300 400 500 600 700R [mm]

1600!

1400!

1200!

1000!

800!

600!

400!

200!

0

z [m

m]

6!10

5!10

4!10

3!10

2!10

1!10

[arb

.]

LZ Preliminary

NR > 3 keVnr

0 100 200 300 400 500 600 700R [mm]

1600!

1400!

1200!

1000!

800!

600!

400!

200!

0

z [m

m]

6!10

5!10

4!10

3!10

2!10

1!10

[arb

.]

LZ Preliminary

ER < 5 keVee

!! Single scattering events in signal region !! Consider NRs near endpoint (4.6 keVnr)

Signal Energy Spectrum in LXe

19

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Visible Energy [keVee]

4!10

3!10

2!10

1!10

1

Even

t Rat

e [C

ts /

keVe

e / s

ec]

Photoneutron NR

Y88 Gamma ER

Photoneutron ER

LZ Preliminary

!! MC events from slide 11 and 12 before detector response effects applied

Pushing to Lower Energies

20

!! S2 only (no S1 detected) events shown, but not the focus of this talk

!! S1+S2 events are mostly > 10 e- "! Confusion with delayed electron

events less likely (at toy MC level)

!! S2 only spectrum needs more detailed simulation to understand delayed electron effect