Neutron captures update Pierre Lasorak, Aran Borkum & Tyler - - PowerPoint PPT Presentation

Neutron captures update

1

Pierre Lasorak, Aran Borkum & Tyler Alion

Pierre Lasorak 20/06/2019

Intro/Content

• Neutrons from the surrounding rock are expected to create 100 Hz of capture in a 10

kT module.

• Been demonstrated that neutron captures is one of the main background for
• SN triggering,
• solar neutrino triggering
• and solar analysis.
• Important to properly characterise this background.
• In this talk:
• Geometry developments
• Water shielding
• Other sources of neutrons
• Neutron spectrum
• Impact on the SN trigger.

2

Pierre Lasorak 20/06/2019

Geometry development

• The 1x2x6 isn’t the best for neutron simulation:
• Neutrons have low cross section on Argon and populate

the whole detector volume, some of them escape the volume.

• Hard to scale the rate from the 1x2x6 to a full 10kT

detector.

• Need to have a more informed analysis if we want to

motivate the addition of water panels.

3

Pierre Lasorak 20/06/2019

Proposed solution

• Overall revamping of the geometry for these studies:
• Utilisation of GGD (for General Geometry Description):
• Provides a simple python interface to the nasty GDML that is used

in ROOT and GEANT4.

• https://github.com/DUNE/duneggd/tree/master/python/duneggd/

larfd

• Integration of the 1x2x6 and 10kT in GGD has been done
• Simple change to the configuration to generate whichever

geometry (number of APA) one wants.

• Creating smaller representative geometry that is easily scalable

to the 10kT.

• Addition of representative cryostat geometry:
• Foam, steel of the cryostat.
• Water panels to test neutron shielding

4

Pierre Lasorak 20/06/2019

Testing

• Quite a lot of tests have been done:
• Up to event display for the 1x2x6:
• 3 wire views are looking OK
• Crossing TPC muons are looking

OK

• 10 kT hasn’t been tested up to

event display (but DetSim is slow…)

5

1x2x6 Full 10kt Water Shielded APAs on Outside Full Cryo, Few APA

G G D

GGD Tests

✔ ✔ ✔ ✔

…

Ma versions easily configured Te th pa to possible config.

ROOT Tests

✔ ✔ ✔ ✔

L A R S O F T

Geometry Sorting

✔ ✔ ✔ ✔

…

Channel Mapping

✔ ✔ ✔ ✔

G4 Overlaps

✔

G4 / IDEs

✔ — ✔ —

DetSim / EVD

✔

Passes existing FCL

✔

… … …

• ns shot down the

e placement seems to

Muon in 3 views Muon crossing the boundary

Pierre Lasorak 20/06/2019

Water shielding results

• Truth study up to GEANT4 stage.
• Test against analysis from John Beacom’s group.
• Similar reduction is observed.

6

es

10kT with water shielding panels

Pierre Lasorak 20/06/2019

Still on the TODO list

• Finish the tests of the geometries.
• Add representative external volumes to get an idea of

neutron shielding.

• Better representation of the cavern.
• Add the photon detectors in the geometry.

7

Pierre Lasorak 20/06/2019

Other sources of neutrons?

• From Paola Sala’s

presentation at CERN.

• Completely different

results!

• Main contributor is the

foam (0.2 Hz)

• 0.06 Hz of capture from

the surrounding rock

• Needs to be investigated.

8

Polyurethane foam (37.6cm) Polyur ethane foam (37.6cm)

LAr

Internal SS (1.2mm)

Plywood (2x1.2cm) + SS (1mm) Plywood (1.2cm) Plywood (1.2cm) External SS membrane (1cm) 80cm

• Mass

[t] Disint/year 10mBq/kg Fission [n/year] Alpha,n [n/year] 10mBq/kg External SS SS460ML 250 7.9e10 71 000 53 490 Foam polyurethane 34 1.1e10 7.5E12 9 600 6.7e6 22 516 Internal SS SS304 4 0.13e10 3 400 1 168 Wood plywood 12 0.4e10 1 200 3 257

Pierre Lasorak 20/06/2019

Neutron spectrum

• Along with the overall neutron rate, the neutron spectrum

is also a subject to discussion:

• Found that there is little difference in the neutron capture

rate in the 10kT with changing spectrum.

• These analyses should probably be repeated with water

shielding.

9

Neutron capture Rates LArSoft 104 Hz NeuCBOT 108 Hz SOURCES4 101 Hz

Pierre Lasorak 20/06/2019

Impact on the SN triggers

• Assuming simple 10 seconds counting

trigger:

• No shape discrimination as shown at

the last CM.

• LMC is much better if you reduce the

neutron background.

10

10

2

10

3

10

Neutron capture rate [Hz]

0.7 0.75 0.8 0.85 0.9 0.95 1

SN trig eff Galaxy far side

1 −

10 1 10

2

10

3

10

Neutron capture rate [Hz]

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

SN trig eff LMC

A P A C P A 3 9 A r g

• n

N e u t r

• n

K r y p t

• n

P

• l
• n

i u m R a d

• n

4 2 A r g

• n

T

• t

a l

0.01 0.02 0.03 0.04 0.05 0.06 0.07

Rate [Hz]

Pierre Lasorak 20/06/2019

Impact on SN trigger

• Using the shape information to inform the trigger decision.
• Principle, simple extension of the counting algorithm:
• Integrate for 10 seconds the sumADC of the clusters
• Compare (likelihood ratio) with the shape of the background.
• If the likelihood exceeds a certain value → trigger.
• Divided the Likelihood into 2 parts:
• Shape → helps when the background is high. Doesn’t

depend on the normalisation of the background.

• At the LMC, the efficiency doesn’t change if the neutron

capture are 100 Hz or more.

• Normalisation (i.e. counting) → is only efficient at low

background.

• Effects turns on when the neutron capture gets around 10 Hz
• Consistent with previous study.
• Unfortunately I didn’t have time to run enough toys to get a

quantifiable answer.

11

20 40 60 80 100 120 140 160 180 200 SADC

10

10

10

10

PDF PDF

Neutron capture rates: 1 kHz 400 Hz 100 Hz 10 Hz 1 Hz

PDF

PDF

PDF

PDF

400 Hz 1 Hz

Pierre Lasorak 20/06/2019

Conclusion

• On-going development with the geometry will be able to inform low-energy triggering studies
• Quantifiable impact of the external volumes (foam, water, shape of the cavern etc) on the

neutron rates.

• Important to cross check and make sure all the potential neutron sources are identified.
• Neutron spectrum shape shouldn’t really matter, all the neutrons bounce elastically and

thermalise before capturing.

• Impact on the trigger:
• Counting only trigger: big impact at the LMC (efficiency goes from 10 % → 50 % if neutron

rate goes from 100 Hz → 10 Hz).

• Shape trigger: effect on the LMC is mitigated at neutron capture rate high rates, more studies

needed to understand what happens at lower rates.

• Future:
• Other backgrounds have been implemented in the radiological generator Bismuth-Polonium

backgrounds.

• Probably worth a request for a production when we are happy with all these new things?

12