ACED - measuring the neutron capture cross-

section of argon

Leon Pickard University of California, Davis

2nd December 2019

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Contents

• Motivation for ACED

• Existing measurements

• ACED setup

• Analysis

• Current and future work

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Deep Underground Neutrino Experiment (DUNE)

• DUNE is a next-generation long-baseline neutrino experiment.

• Its near and far detectors consist of liquid argon time projection chambers.

• It has an international team of over 1000 collaborators from 30 countries.

• Goals are wide and varied, including measurements of:

• CP violation

• mass hierarchy

• supernovae

• proton decay

• …

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DUNE can observe supernovae

• Core-collapse supernovae produce and emit ~1058 neutrinos. • Only once (from SN1987A) have supernovae neutrinos been observed. • DUNE offers:

• Unique sensitivity to νe’s in the ~10s MeV regime • High-statistics observations • Understanding of supernovae dynamics • Quantitative supernovae model validation • Neutrino magnetic moment measurement • …

Simulated time-, and energy-, dependent signal in the 40 kt DUNE far detector for a supernova 10 kpc away

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• Calculations performed by S.Gardiner (UC Davis/Fermilab) predicts that supernovae events in DUNE often produce final state neutrons.

• Neutron capture cross section is known poorly - thus predicting when the capture will occur is difficult.

• The gamma cascade following capture isn't fully known - thus tagging the capture is difficult.

• To improve modeling, and supernovae neutrino detection efficacy, we need to know both of these well.

• How will such events behave in DUNE:

1. Neutron escapes nuclear environment.

2. Neutron scatters within the detector…

3.…eventually capturing on argon.

• If this capture is missed…the supernova neutrino energy is mis-reconstructed!

Motivation for ACED - Supernovae

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Existing neutron capture measurements

Neutron energy (MeV)

Cross section (barns)

Uncertainty (barns) Year Author

2.53 x 10-8 0.51 0.025 1969 N.Ranakumar et al

2.53 x 10-8 0.723 0.025 1965 R.L.D. French et al

2.53 x 10-8 0.63 0.02 1963 W. Koehler et al

5 x 10-3 3.2 x 10-3 1.8 x 10-4 1989 R.L. Macklin et al

1.751 x 10-2 1.702 x 10-2 2.442 x 10-2 1989 R.L. Macklin et al

0.136 2.2 x 10-3 8 x 10-4 1959 N.A. Bostrom et al

0.37 1.19 x 10-3 6.00 x 10-5 2014 M. Bhike et al

0.03 2.54 x 10-3 1 x 10-4 2000 Z.Y. Bao et al

0.03 2.45 x 10-3 0 2006 S.F. Mughabghab

0.0234 2.55 x 10-3 1.5 x 10-4 2002 H. Beer et al

Table compiled by J. Wang

Very few thermal neutron capture measurements:

• All performed 50+ years ago!

• Much tension between the three measurements

• ACED aims to resolve this

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ACED: Argon Capture Experiment at DANCE

• Goals:

• Measure the thermal neutron capture cross section on 40Ar

• Model the nuclear energy levels of 41Ar

UC Davis - S. Gardiner, V. Fischer, J. He, T. Johnson, L. Pagani, L. Pickard, E. Pantic, R. Svoboda and J. Wang

LANL - A. Couture, C. Prokop and J. Ullmann

Boston University - C. Grant

The A(CED)-Team

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DANCE: Detector for Advanced Neutron Capture Experiments

• Located in Flight Path 14 of the Lujan Neutron Scattering Centre (LANSCE).

• 20.25 m from the water moderator of the accelerator-driven neutron source.

• Neutrons produced by impinging 800 MeV protons onto a tungsten target.

• DANCE is a ~4π gamma ray calorimeter: • Spherical array of 160 BaF2 crystals • Segmentation allows measurement

of γ-multiplicity • Each monitored by a PMT • 6 cm thick 6LiH shell attenuates

scattered neutrons capturing on the BaF2 crystals

• Time-of-flight methodology used to measure neutron energy with a few MeV accuracy.

• Downstream monitors measure and characterise the neutron beam flux.

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Analysis - Background subtraction requires three beam runs

• Three data sets used for background subtraction:

• Beam incident on Ar (A) - To measure neutron captures

• No beam incident on Ar (S) - To understand non-beam related backgrounds

• Beam incident on vacuum (V) - To understand backgrounds produced by the beam and target setup

Number of neutron captures on Argon

Neutron flux

Neutron capture on argon

Number of events

Number of beam spills

ACED target volume

ACED gas system

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Analysis - Cross section determination methodology

Cuts: • 0.02 < Neutron energy < 0.04 eV • 5.2 < Q-value < 6.6 MeV • # of clusters > 1

Neutron beam monitor flux measurementsEnergy spectra of individual clusters for the three data

sets and the resulting background subtracted data

Lithium monitor

Helium monitor

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ACED has measured the thermal neutron capture cross section

40Ar capture cross section as a function of neutron velocity

Fitting the ACED data for 40Ar capture as a function of neutrino velocity (and correcting for the temperature of the moderator and target), yields a thermal neutron capture cross section of:

σ = 673 ± 26 (stat.) ± 59 (sys.) mb

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Next step - Measuring the gamma cascade using DICEBOX

• Now we have measured the thermal neutron capture cross section on 40Ar, is it possible to determine the gamma cascade of 41Ar?

• Many subtleties and difficulties in doing so (e.g. understanding detector response, numerous unknown high-lying levels, parameterising the efficacy of the background subtraction method, etc).

We know the underlying nuclear energy levels of 41Ar. This is used as a starting point for the DICEBOX input.

DICEBOX, using: this information, known statistical distributions of level-spacings, gamma intensities, and nuclear models, produces “nuclear realisations"

Simulate each realisation in DANCE. This can be compared to our data to find the “winning” model and parameterisation.

The gamma cascade analysis is a 3-step procedure:

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Preliminary results…

• The DICEBOX output can be optimised to find the best match with the data by using different:

• nuclear models

• input parameters

• nuclear realisations

• Analysis is ongoing…

Data

MC

Data

MC

Data

MC

Eve

nts

per b

in (A

.U.)

Eve

nts

per b

in (A

.U.)

The ugly

The bad The good

Eve

nts

per b

in (A

.U.)

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Conclusions

• An improved understanding of the thermal neutron capture on argon will help the next generation of physics experiments

• Previous measurements are sparse and in disagreement with one another

• The ACED experiment, using the LANSCE facility, has successfully measured the thermal neutron capture cross section on 40Ar

• σ = 673 ± 26 (stat.) ± 59 (sys.) mb

• Further understanding is ongoing. The gamma cascade analysis to determine the nuclear structure of 41Ar is underway.