pith. sign in

arxiv: 1907.04322 · v1 · pith:HN6VLLOInew · submitted 2019-07-09 · ⚛️ physics.ins-det · nucl-ex

Monte Carlo calculation of the average neutron depolarization for the NPDGamma experiment

Kyle B. Grammer , James D. Bowman This is my paper

Pith reviewed 2026-05-25 00:04 UTC · model grok-4.3

classification ⚛️ physics.ins-det nucl-ex
keywords neutron depolarizationMonte Carlo simulationNPDGammaparahydrogen targetspin trackingMCNPXpolarized neutron capture
0
0 comments X

The pith

Monte Carlo spin tracking calculates average neutron polarization at capture to correct the NPDGamma beam measurement.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The NPDGamma experiment measures gamma-ray asymmetry from polarized neutrons captured on liquid parahydrogen, but the upstream helium-3 polarization measurement misses depolarization inside the target. Residual orthohydrogen causes spin-flip scattering that reduces the effective polarization at the moment of capture and therefore dilutes the observed asymmetry. This paper implements neutron spin tracking inside the MCNPX Monte Carlo code to follow the spin state of each neutron through all scattering events and compute the ensemble-averaged polarization remaining at capture. The resulting average supplies a multiplicative correction factor applied to the measured beam polarization. The approach treats the target geometry and scattering cross sections explicitly so the correction reflects the actual conditions of the experiment.

Core claim

Methods for tracking neutron spin in MCNPX were developed in order to calculate the average neutron polarization upon capture for use as a multiplicative correction to the measured beam polarization for the NPDGamma experiment.

What carries the argument

Neutron spin tracking inside MCNPX, which propagates the spin vector of each simulated neutron through elastic and spin-flip scattering until capture and then averages the final polarization over the ensemble.

If this is right

  • The measured gamma asymmetry must be divided by the calculated average polarization to recover the true asymmetry value.
  • The correction accounts only for depolarization inside the parahydrogen target and is applied after the upstream beam polarization measurement.
  • The same spin-tracking technique can be reused for any future polarized-neutron experiment that uses a liquid-hydrogen target of similar composition.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Running the simulation with varying orthohydrogen concentrations would map how sensitive the correction is to target purity.
  • Adding magnetic-field tracking to the same code would let the method handle experiments that apply holding fields inside the target.
  • The resulting polarization map could be used to optimize target design for future asymmetry measurements.

Load-bearing premise

Depolarization is caused mainly by spin-flip scattering from orthohydrogen in the bulk liquid and the Monte Carlo model includes every important process without significant missing contributions.

What would settle it

Compare the Monte Carlo average polarization directly to a measurement of polarization performed at the capture location inside the target; a statistically significant mismatch would show the correction factor is incorrect.

Figures

Figures reproduced from arXiv: 1907.04322 by James D. Bowman, Kyle B. Grammer.

Figure 1
Figure 1. Figure 1: Neutron scattering cross sections on parahydroge [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Parahydrogen scattering kinematics from MCNPX [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Orthohydrogen scattering kinematics from MCNPX [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Input neutron spectrum used in the MCNPX cal [PITH_FULL_IMAGE:figures/full_fig_p007_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: ∆r dep for neutrons that capture on aluminum 6061 containing cells as a function of parahydrogen concentration and shown for each detector. Ring ∆r dep(xo,lower) ∆r dep(xo,upper) 1 0.9961(3) 0.9799(8) 2 0.9955(3) 0.9722(8) 3 0.9939(5) 0.9581(13) 4 0.9941(6) 0.9556(19) Total 0.9953(2) 0.9724(5) [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: Polarization correction for each ring as a functio [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
read the original abstract

The NPDGamma experiment measures the asymmetry in $\gamma$-ray emission in the capture of polarized neutrons on liquid parahydrogen. The beam polarization is measured using $^3$He spin analysis, but this measurement does not account for the contribution of depolarization from spin-flip scattering primarily due to orthohydrogen in the bulk liquid. This is a systematic effect that dilutes the experimental asymmetry and is modeled using Monte Carlo. Methods for tracking neutron spin in MCNPX were developed in order to calculate the average neutron polarization upon capture for use as a multiplicative correction to the measured beam polarization for the NPDGamma experiment.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 0 minor

Summary. The manuscript describes the development of neutron spin-tracking extensions to the MCNPX Monte Carlo code. These are applied to compute the average neutron polarization at capture in the liquid parahydrogen target of the NPDGamma experiment, yielding a multiplicative correction factor for depolarization arising primarily from orthohydrogen spin-flip scattering.

Significance. If the simulation is shown to be reliable, the work supplies a targeted computational correction for a key systematic in a precision neutron-capture asymmetry measurement. The approach is experiment-specific rather than a general theoretical advance.

major comments (1)
  1. [Abstract] Abstract: the description states that spin tracking was added to MCNPX and used to compute an average polarization, but provides no validation data, comparison to analytic limits, or error estimates; this prevents assessment of the soundness of the central result beyond the high-level description.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their review of our manuscript on the Monte Carlo calculation of average neutron depolarization for the NPDGamma experiment. We address the single major comment below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the description states that spin tracking was added to MCNPX and used to compute an average polarization, but provides no validation data, comparison to analytic limits, or error estimates; this prevents assessment of the soundness of the central result beyond the high-level description.

    Authors: We agree that the abstract as written does not mention validation, analytic comparisons, or error estimates. The manuscript body contains these elements, including direct comparisons of the spin-tracking implementation to analytic limits for uniform fields and simple scattering cases, plus statistical uncertainties from the Monte Carlo runs and estimated systematic uncertainties from orthohydrogen concentration. To address the referee's concern and allow assessment from the abstract, we will revise the abstract to include a concise statement on the validation performed and the final uncertainty on the depolarization correction factor. revision: yes

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper describes development of neutron spin-tracking extensions in MCNPX to compute average polarization at capture via Monte Carlo simulation of spin-flip scattering from orthohydrogen. This output is presented as an independent simulation result used as a multiplicative correction, with no equations, fitted parameters, or self-citations that reduce the reported polarization to an input by construction. The central claim rests on the simulation's physics implementation rather than any self-definitional or tautological step.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review supplies no explicit free parameters, axioms, or invented entities; the simulation implicitly relies on standard neutron cross sections and the assumption that orthohydrogen scattering dominates depolarization.

pith-pipeline@v0.9.0 · 5626 in / 1056 out tokens · 20518 ms · 2026-05-25T00:04:14.901225+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

31 extracted references · 31 canonical work pages

  1. [1]

    Blyth, J

    D. Blyth, J. Fry, N. Fomin, R. Alarcon, L. Alonzi, E. Askanazi, S. Baeßler, S. Balas- cuta, L. Barr´ on-Palos, A. Barzilov, J. D. Bow- man, N. Birge, J. R. Calarco, T. E. Chupp, V. Cianciolo, C. E. Coppola, C. B. Craw- ford, K. Craycraft, D. Evans, C. Fieseler, E. Frleˇ z, I. Garishvili, M. T. W. Gericke, R. C. Gillis, K. B. Grammer, G. L. Greene, J. Hall...

    work page 2018

  2. [2]

    M. T. Gericke, R. Alarcon, S. Balascuta, L. Barr´ on-Palos, C. Blessinger, J. D. Bowman, R. D. Carlini, W. Chen, T. E. Chupp, C. Craw- ford, S. Covrig, M. Dabaghyan, N. Fomin, S. J. Freedman, T. R. Gentile, R. C. Gillis, G. L. Greene, F. W. Hersman, T. Ino, G. L. Jones, B. Lauss, M. Leuschner, W. R. Lozowski, R. Mahurin, Y. Masuda, J. Mei, G. S. Mitchell,...

    work page 2011

  3. [3]

    Fomin, G

    N. Fomin, G. L. Greene, R. R. Allen, V. Cian- ciolo, C. Crawford, T. M. Tito, P. R. Huffman, E. B. Iverson, R. Mahurin, W. M. Snow, Fun- damental neutron physics beamline at the spal- lation neutron source at ORNL, Nucl. Instrum. Methods Phys. Res., Sect. A 773 (2015) 45–51

    work page 2015

  4. [4]

    Balascuta, R

    S. Balascuta, R. Alarcon, S. Baeßler, G. Greene, A. Mietke, C. Crawford, R. Milburn, S. Penttila, J. Prince, J. Sch¨ adler, The implementation of a super mirror polarizer at the SNS fundamental neutron physics beamline, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and As- sociated Equipment 671 (20...

    work page 2012

  5. [5]

    M. T. Gericke, J. D. Bowman, R. D. Carlini, T. E. Chupp, K. P. Coulter, M. Dabaghyan, M. Dawkins, D. Desai, S. J. Freedman, T. R. Gentile, R. C. Gillis, G. L. Greene, F. W. Hers- man, T. Ino, G. L. Jones, M. Kandes, B. Lauss, M. Leuschner, W. R. Lozowski, R. Mahurin, M. Mason, Y. Masuda, G. S. Mitchell, S. Muto, H. Nann, S. A. Page, S. I. Penttil¨ a, W. D...

    work page 2006

  6. [6]

    J. Fry, R. Alarcon, R. Allen, E. Askanazi, S. Bal- ascuta, L. Barron-Palos, S. Baeßler, A. Barzilov, C. Blessinger, D. Blyth, J. D. Bowman, J. R. Calarco, T. E. Chupp, C. E. Coppola, C. Craw- ford, K. Craycraft, M. Dabaghyan, D. Evans, J. Favela, C. Fieseler, N. Fomin, W. Fox, S. Freedman, E. Frleˇ z, C. Fu, C. Garcia, I. Gar- ishvili, M. T. Gericke, R. C...

    work page

  7. [7]

    P.-N. Seo, L. Barr´ on-Palos, J. D. Bowman, T. E. Chupp, C. Crawford, M. Dabaghyan, M. Dawkins, S. J. Freedman, T. Gentile, M. T. Gericke, R. C. Gillis, G. L. Greene, F. W. Hers- man, G. L. Jones, M. Kandes, S. Lamoreaux, B. Lauss, M. B. Leuschner, R. Mahurin, M. Ma- son, J. Mei, G. S. Mitchell, H. Nann, S. A. Page, S. I. Penttil¨ a, W. D. Ramsay, A. Sala...

    work page 2008

  8. [8]

    Santra, L

    S. Santra, L. Barrn Palos, C. Blessinger, J. D. Bowman, T. E. Chupp, S. Covrig, C. Craw- ford, M. Dabaghyan, J. Dadras, M. Dawkins, M. T. Gericke, W. Fox, R. C. Gillis, M. B. Leuschner, B. Lozowski, R. Mahurin, M. Ma- son, J. Mei, H. Nann, S. I. Penttila, A. Salas- Bacci, M. Sharma, W. M. Snow, W. S. Wilburn, A liquid parahydrogen target for the measure- ...

    work page 2010

  9. [9]

    M. T. Gericke, C. Blessinger, J. D. Bowman, R. C. Gillis, J. Hartfield, T. Ino, M. Leuschner, Y. Masuda, G. S. Mitchell, S. Muto, H. Nann, S. A. Page, S. I. Penttil¨ a, W. D. Ramsay, P.- N. Seo, W. M. Snow, J. Tasson, W. S. Wilburn, A current mode detector array for γ ray asym- metry measurements, Nuclear Instruments and Methods in Physics Research, Sectio...

    work page 2005

  10. [10]

    Musgrave, S

    M. Musgrave, S. Baeßler, S. Balascuta, L. Barr´ on-Palos, D. Blyth, J. Bowman, T. Chupp, V. Cianciolo, C. Crawford, K. Cray- craft, N. Fomin, J. Fry, M. Gericke, R. Gillis, K. Grammer, G. Greene, J. Hamblen, C. Hayes, P. Huffman, C. Jiang, S. Kucuker, M. McCrea, P. Mueller, S. Penttil¨ a, W. Snow, E. Tang, Z. Tang, X. Tong, W. Wilburn, Measurement of the a...

    work page 2018

  11. [11]

    J. J. Szymanski, J. D. Bowman, P. P. J. Delheij, C. M. Frankle, J. Knudson, S. Penttil¨ a, S. J. See- strom, S. H. Yoo, V. W. Yuan, X. Zhu, Ion cham- ber system for neutron flux measurements, Nucl. Instrum. Methods Phys. Res., Sect. A 340 (3) (1994) 564–571

    work page 1994

  12. [12]

    D. M. Dennison, A Note on the Specific Heat of the Hydrogen Molecule, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 115 (771) (1927) 483–486

    work page 1927

  13. [13]

    Barr´ on-Palos, R

    L. Barr´ on-Palos, R. Alarcon, S. Balascuta, C. Blessinger, J. D. Bowman, T. E. Chupp, S. Covrig, C. B. Crawford, M. Dabaghyan, J. Dadras, M. Dawkins, W. Fox, M. T. Gericke, R. C. Gillis, B. Lauss, M. B. Leuschner, B. Lo- zowski, R. Mahurin, M. Mason, J. Mei, H. Nann, S. I. Penttil¨ a, W. D. Ramsay, A. Salas-Bacci, S. Santra, P. N. Seo, M. Sharma, T. Smit...

    work page 2011

  14. [14]

    Louis, MO 63103, USA, Product Number 371254-250G

    Sigma-Aldridge Corp., 3050 Spruce Street, St. Louis, MO 63103, USA, Product Number 371254-250G. 11

    work page

  15. [15]

    Chadwick, M

    M. Chadwick, M. Herman, P. Obloˇ zinsk´ y, M. Dunn, Y. Danon, A. Kahler, D. Smith, B. Pritychenko, G. Arbanas, R. Arcilla, R. Brewer, D. Brown, R. Capote, A. Carl- son, Y. Cho, H. Derrien, K. Guber, G. Hale, S. Hoblit, S. Holloway, T. Johnson, T. Kawano, B. Kiedrowski, H. Kim, S. Kunieda, N. Lar- son, L. Leal, J. Lestone, R. Little, E. Mc- Cutchan, R. Mac...

    work page 2011

  16. [16]

    S. F. Mughabghab, Atlas of Neutron Reso- nances, 5th Edition, Elsevier, New York, 2006

    work page 2006

  17. [17]

    K. B. Grammer, R. Alarcon, L. Barron-Palos, D. Blyth, J. D. Bowman, J. Calarco, C. Craw- ford, K. Craycraft, D. Evans, N. Fomin, J. Fry, M. Gericke, R. C. Gillis, G. L. Greene, J. Ham- blen, C. Hayes, S. Kucuker, W. S. Wilburn, Mea- surement of the scattering cross section of slow neutrons on liquid parahydrogen from neutron transmission, Phys. Rev. B 180...

    work page 2015

  18. [18]

    D. B. Pelowitz, MCNPX User’s Manual, Los Alamos National Laboratory, 2011

    work page 2011

  19. [19]

    Young, J

    J. Young, J. Koppel, Slow Neutron Scattering by Molecular Hydrogen and Deuterium, Phys. Rev. 135 (3A) (1964) A603—-A611

    work page 1964

  20. [20]

    R. Moon, T. Riste, W. Koehler, Polarization Analysis of Thermal-Neutron Scattering, Phys. Rev. 181 (2) (1969) 920–931

    work page 1969

  21. [21]

    Zoppi, Neutron scattering of homonuclear diatomic liquids, Physica B: Condensed Matter 183 (3) (1993) 235–246

    M. Zoppi, Neutron scattering of homonuclear diatomic liquids, Physica B: Condensed Matter 183 (3) (1993) 235–246

    work page 1993

  22. [22]

    Celli, N

    M. Celli, N. Rhodes, A. K. Soper, M. Zoppi, The total neutron cross section of liquid para- hydrogen, J. Phys.: Condens. Matter 11 (50) (1999) 10229–10242

    work page 1999

  23. [23]

    Keinert, J

    J. Keinert, J. Sax, Investigation of neutron scat- tering dynamics in liquid hydrogen and deu- terium for cold neutron sources, Kerntechnik 51 (1987) 19

    work page 1987

  24. [24]

    R. E. MacFarlane, Cold-moderator scattering kernel methods, Tech. rep., Los Alamos Na- tional Laboratory (LANL), Los Alamos, NM (aug 1998)

    work page 1998

  25. [25]

    R. J. Elliott, W. M. Hartmann, Theory of neu- tron scattering from liquid and solid hydrogen, Proceedings of the Physical Society 90 (3) (1967) 671–680

    work page 1967

  26. [26]

    V. F. Sears, Neutron scattering lengths and cross sections, Neutron News 3 (3) (1992) 26–37

    work page 1992

  27. [27]

    Avenier, G

    M. Avenier, G. Bagieu, H. Benkoula, J. F. Cavaignac, A. Idrissi, D. H. Koang, B. Vignon, R. Wilson, Parity non-conservation in the radia- tive capture of polarized neutrons by35Cl, Nu- clear Physics, Section A 436 (1) (1985) 83–92

    work page 1985

  28. [28]

    Vesna, ´E

    V. Vesna, ´E. Kolomenski, V. Lobashev, V. Nazarenko, A. Pirozhkov, L. Smotritski, Y. Sobolev, N. Titov, Observation of parity non- conservation in the integrated γ spectrum from (n, γ) reactions in Cl, Br, Cd, Sn, and La nuclei, Soviet Journal of Experimental and Theoretical Physics Letters 36 (1982) 209

    work page 1982

  29. [29]

    G. S. Mitchell, C. S. Blessinger, J. D. Bowman, T. E. Chupp, K. P. Coulter, M. Gericke, G. L. Jones, M. B. Leuschner, H. Nann, S. A. Page, S. I. Penttil¨ a, T. B. Smith, W. M. Snow, W. S. Wilburn, A measurement of parity-violating gamma-ray asymmetries in polarized cold neu- tron capture on 35Cl, 113Cd, and 139La, Nu- clear Instruments and Methods in Phys...

    work page 2004

  30. [30]

    Meija, T

    J. Meija, T. B. Coplen, M. Berglund, W. A. Brand, P. De Bi` evre, M. Gr¨ oning, N. E. Holden, J. Irrgeher, R. D. Loss, T. Walczyk, T. Pro- haska, Isotopic compositions of the elements 2013 (IUPAC Technical Report), Pure and Ap- plied Chemistry 88 (3) (2016) 293–306

    work page 2013

  31. [31]

    Grammer, D

    K. Grammer, D. Blyth, J. Bowman, N. Fomin, G. Greene, M. Musgrave, E. Tang, Z. Tang, Monte Carlo calculation and verification of the geometrical factors for the NPDGamma exper- iment, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spec- trometers, Detectors and Associated Equipment 903 (2018) 302–308. 13

    work page 2018