External Backgrounds (s) in partial-fill and the Rope Analysis

External Backgrounds (γs) in partial-fill and the RopeAnalysis

Pouya Khaghani

August 13, 2021

Pouya Khaghani (LU) External Backgrounds (γs) in partial-fill and the Rope AnalysisAugust 13, 2021 1 / 17

PhD candidate,Laurentian University/SNO+

Weekend line cook,Motley Kitchen.


The SNO+ Experiment

2km Underground at SNOLABin Vale’s Creighton Mine (∼70cosmic muons/day)

Acrylic vessel (AV): 6m radius,5cm thickness

9400 Photo Multiplier Tubes(PMTs), ∼8m radius PSUP

905 tonnes of ultra-pure water/780 tonnes of liquid scintillatorbased cocktail

7000 tonnes external watershielding.


What is SNO+ searching for?

Physics GoalsWater Phase: 8B SolarNeutrinos and Invisible nucleondecay1.

Pure Scintillator Phase: Solarneutrinos, Geo/Reactoranti-neutrinos, and studyinternal backgrounds.

Tellurium-loaded Scintillator:Neutrino-less double betadecay with 130Te

All phases:

Supernova NeutrinosReactor- and Geo-antineutrinos

1see Physical Review D 99 (1), 012012 & Physical Review D 99 (3), 032008).Pouya Khaghani (LU) External Backgrounds (γs) in partial-fill and the Rope AnalysisAugust 13, 2021 3 / 17

Current Status

Water phase data taking 2017-2019 - In addition to Physics andcalibration, SNO+ made a measurement of external backgrounds(γ)s.Partial-fill 2020 APR-NOV: Filling process stopped due toCOVID-19 for 7 months while the detector was half-full.SNO+ used the partial-fill data to study internal/externalbackgrounds in scintillator.Detector is full and SNO+ is about to start the scintillator phase.

1The figures were taken on April 28th


External γs from 208Tl and 214Bi and 40K

External Backgrounds: High energy γs from Tl208, Bi214 and K40which are generated outside of the scintillator volume (AV) but canpropagate and reconstructed inside.

They come from the AV. external water, HU/HD ropes, PMTs, ...

Tl208 generate γs up to 2.6 MeV which can be background for 0νββand solar signals.

0 0.5 1 1.5 2 2.5 3 3.5 4True MC Energy (MeV)

0

2000

4000

6000

8000

10000

12000

14000

ev

en

ts

dep_energy True energy of the gammas from Tl208_AV (MC simulation)

gammas from Bi214_AV (MC simulation)


Identification of the 2.6 MeV γs from external 208Tl inpartial-fill

The major background for this signal are the 214Bi and 212Bi β decaysinside the detector.

Bi-Po events are tagged using the delayed coincidence taggingtechnique.

A set of MC was generated including the external signals as well as214Bi-Po in partial fill with the interface at 0.8 m and 0.5 g/L PPO.

400 500 600 700 800 900 1000Nhits

0

100

200

300

400

500

600

700

800

even

ts

BiPo214MC_bipo214_MC_partial

BiPo214 evts

External Tl208 gammas

400 500 600 700 800 900 1000Nhits

0

1000

2000

3000

4000

5000

6000

7000

8000

ev

en

ts Tl208 AV

Tl208 HD

Tl208 HD+AV

Bi214 AV

sum external


214BiPo rejection efficiency and the external sacrifice

99% of the externals are being reconstructed beyond R > 5.2maccording to MC.

ROI: 5.2m< R & 2m< ρ (to avoid PFA tubing) & 1.5 < z < 5.3m

For rope studies the same cuts were used plus 1.5 < z < 3m

1000 2000 3000 4000 5000 6000R [mm]

1

10

210

310even

ts

Pos R[mm]

gammas from Tl208_AV (MC) 0.5g/L PPO

BiPo214 in scintillator (MC)

gammas from Bi214_AV (MC)


Tagging and rejecting 214BiPo events for 4.2m < R between 20200401 and

20200808 ( 70 days, most of June/July data are not included!)

nhits_PoEntries 41000Mean 239.8Std Dev 21.7

200 300 400 500 600 700 800 900 1000nhits

0

500

1000

1500

2000

2500

3000

3500

4000

nhits_PoEntries 41000Mean 239.8Std Dev 21.7

1500 2000 2500 3000 3500 4000 4500 5000rho [mm]

2000

2500

3000

3500

4000

4500

5000

5500

6000

Z [m

m]

0

50

100

150

200

250

300

0 100 200 300 400 500 600 700 800 900 1000delta R [mm]

0

200

400

600

800

1000

0 200 400 600 800 1000 1200 1400 1600 1800 2000

310×

delta t [ns]

0

200

400

600

800

1000

1200

1400

1600


Removing BiPo214/212 events

nhits_NoBiPosEntries 5379441

Mean 223.2

Std Dev 28.72

200 300 400 500 600 700 800 900 1000Nhits

1

10

210

310

410

510

ev

en

ts

nhits_NoBiPosEntries 5379441

Mean 223.2

Std Dev 28.72

nhits (removed BiPos)

Tl208K40

Bi214/(gammas)

Bi210


Removing BiPo214/212 events and identifying 208Tl signal

The fit values: µ = 762 ± 86 consistent with the calibration lightcurve.

Light yield has a significant radial dependency.

nhits_Tl208Entries 26293

Mean 728.2Std Dev 85.54

600 650 700 750 800 850 900 950 10000

100

200

300

400

500

600

nhits_Tl208Entries 26293

Mean 728.2Std Dev 85.54

ExTl208_like_nhits

No upper R cut R<6m R<5.95m R<5.90m R<5.85m R<5.8m


Reconstructed position and angular distribution

Total events with a validfitter:

6000− 4000− 2000− 0 2000 4000 6000x [mm]

6000−

4000−

2000−

0

2000

4000

6000

y [m

m]

0

5

10

15

20

25

30

35

40

1.5− 1− 0.5− 0 0.5 1 1.5theta

500

550

600

650

700

750

800

850

900

after tagging and rejectingBiPo214/212s:

6000− 4000− 2000− 0 2000 4000 6000pos x [mm]

6000−

4000−

2000−

0

2000

4000

6000

pos

y [m

m]

0

10

20

30

40

50

60

6000− 4000− 2000− 0 2000 4000 6000Pos x [mm]

6000−

4000−

2000−

0

2000

4000

6000

Pos

y [m

m]


Azimuthal analysis of the ropes through stacking method

Rope Analysis cuts: 1.5 < Z < 3m & 5.2m < R & energy > 2.6MeV.

Stacking method done is used to measure the level of backgroundfrom the HD ropes. (see SNO+-doc-5169-v1).


https://www.snolab.ca/snoplus/private/DocDB/cgi/ShowDocument?docid=5169

Month by month Azimuthal analysis of data(20200401-20200920)

Stack_Theta_Tl208_750

Entries 2091Mean 8.951Std Dev 4.782

0 2 4 6 8 10 12 14 16 18Phi

160

180

200

220

240

260

280



events vs. azimuthal angle (E>2.6 MeV)

April livetime = 713hrs



0 2 4 6 8 10 12 14 16 18Phi

120

140

160

180

200

220




July 705 hrs



0 2 4 6 8 10 12 14 16 18Phi

40

50

60

70

80

90




May 370.3 hrs



0 2 4 6 8 10 12 14 16 18Phi

80

100

120

140

160




August 662 hrs



0 2 4 6 8 10 12 14 16 18Phi

40

50

60

70

80

90

100

110




June 470.4 hrs



0 2 4 6 8 10 12 14 16 18Phi

40

50

60

70

80

90

100

110Stack_Theta_Tl208_750



September448.4 hrs


Conclusions and the next steps

The 2.6 MeV γs is identified in partialfill, and used for calibrationpurposes.

The γs from the ropes is estimated in partialfill and (data/MC) isconsistent with previous measurements. data/MC ∼0.31 ±0.04(doc-6580-v1).

In order to reject the externals several classifiers are developed andoptimized based on timing and geometry of the events.


https://www.snolab.ca/snoplus/private/DocDB/0065/006580/001/Rope_Analysis_plots.pdf

MC production for partial-fill and expected rate of externalbackgrounds.

RAT-6.18.7

source Tl208 AV Tl208 HD K40 AV K40 HD K40 water Bi214 AV Bi214 HDPMT gammasTl208

Expectednumber ofevts/year

1.5e+06(2.32/1.55)e+6

(full/shell)7.32e+7 1.89e+8 3.92e+06 2.67e+6

(4.06/2.72)e+6

(full/shell)—-

Simulatedevts.

614248 580637 3044471 5442269 – 1130866 542112 500458

Scale factor(roughly 70 days)

0.5856 0.64021 — 0.2046 2.7923

No. of evtswith valid fit

165100

No, eventsRope cuts**with E>2.6 MeVNhit>600 (MC)

2057


Backup; BiPo214 tagging


Backup; BiPo212 tagging


External Backgrounds (s) in partial-fill and the Rope Analysis

Documents

Transcript of External Backgrounds (s) in partial-fill and the Rope Analysis