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arxiv: 1907.05413 · v1 · pith:MM35SJQZnew · submitted 2019-07-11 · ⚛️ nucl-ex

Two Neutron Correlation Study in Photofission of Actinides

Roman V. Shapovalov This is my paper

Pith reviewed 2026-05-24 22:20 UTC · model grok-4.3

classification ⚛️ nucl-ex
keywords two-neutron correlationphotofissionactinidesangular asymmetryfission fragmentsneutron emissiondetection technique
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0 comments X

The pith

Two neutrons from photofission are emitted mostly in opposite directions.

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

The paper proposes to measure the angular correlation between pairs of neutrons produced in the photofission of actinides. It starts from the established fact that the two fission fragments fly apart back to back and assumes standard neutron emission in each fragment's rest frame. A preliminary calculation finds a large asymmetry in which anti-parallel neutron pairs greatly outnumber parallel pairs, and the asymmetry grows when an energy threshold is applied to each neutron. Confirmation would open a route to detecting actinides by counting neutrons rather than the harder-to-detect fragments.

Core claim

The two neutron correlation shows a huge asymmetry effect with many more neutrons emitted anti-parallel to each other than parallel to each other. That asymmetry becomes even more if the energy cut on each neutron is done. This arises because the fission fragments are emitted essentially back to back in the laboratory frame.

What carries the argument

The mapping of neutron emission directions from the back-to-back fission fragment rest frames into the laboratory frame.

If this is right

  • This asymmetry can serve as a unique signature of fissionable materials.
  • Neutron measurements can carry information about fission fragment energy and angular spectra.
  • The study can improve knowledge of correlated neutron emission.
  • It may enable new techniques for actinide detection in homeland security and safeguards.

Where Pith is reading between the lines

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

  • The proposed measurement could be performed with existing neutron detector arrays without requiring fragment detection.
  • Different actinide targets or photon energies might produce measurable differences in the asymmetry strength.

Load-bearing premise

That the known back-to-back emission of fission fragments, combined with standard assumptions about neutron emission in the fragment rest frame, is sufficient to produce a reliable quantitative prediction of the lab-frame two-neutron angular correlation without experimental validation or detailed modeling of the photofission process.

What would settle it

An experimental measurement of the two-neutron angular distribution in photofission that finds equal numbers of neutron pairs at 0 degrees and 180 degrees.

Figures

Figures reproduced from arXiv: 1907.05413 by Roman V. Shapovalov.

Figure 1.2
Figure 1.2. Figure 1.2: The average TKE as a function of the heavy frag￾ment mass. The solid line is the result of a least-square fit￾ting of the experimental data sets [17] [PITH_FULL_IMAGE:figures/full_fig_p006_1_2.png] view at source ↗
Figure 1.3
Figure 1.3. Figure 1.3: Integrated fission fragments yield (arbitrary units) versus fragment mass for the [PITH_FULL_IMAGE:figures/full_fig_p006_1_3.png] view at source ↗
Figure 2.1
Figure 2.1. Figure 2.1: The angular distribution, N(Θ), of fission fragments from Th232 caught at the angles Θ to the x-ray beam. The x-ray beam was produced in a thick lead target by an electron beam whose spectrum was centered at 13 MeV and was about 5 MeV wide [9] [PITH_FULL_IMAGE:figures/full_fig_p009_2_1.png] view at source ↗
Figure 2.3
Figure 2.3. Figure 2.3: The ratio of measured to calculated values for (a) numbers of neutrons (b) average [PITH_FULL_IMAGE:figures/full_fig_p010_2_3.png] view at source ↗
Figure 2.5
Figure 2.5. Figure 2.5: Fission neutron energy spectrum divided by the square root of the neutron en￾ergy versus the neutron energy. The solid line is Maxwell energy distribution. in the fission fragment rest frame integrated over all neutron energies and normalized to unity is plotted in [PITH_FULL_IMAGE:figures/full_fig_p011_2_5.png] view at source ↗
Figure 2.6
Figure 2.6. Figure 2.6: Fission neutron angular distribu￾tion in the fragment center-of-mass system in￾tegrated over all neutron energies [PITH_FULL_IMAGE:figures/full_fig_p011_2_6.png] view at source ↗
Figure 3.1
Figure 3.1. Figure 3.1: Typical TOF spectrum from photodisintegration of deuteron measured from previous HRRL runs. The distance from target to detector is about 2 m. The spectrum illustrate the ability to distin￾guish gammas peak from neutrons one [PITH_FULL_IMAGE:figures/full_fig_p012_3_1.png] view at source ↗
Figure 3.2
Figure 3.2. Figure 3.2: Possible detector geometry to measure the two neutron correlation yield. Total 16 neutron detectors are placed at the angle of 90 degree with re￾spect to the beam. The detector size is 15 cm × 88 cm × 3.8 cm. detector located 1 m away from the target, that will correspond to the TOF equal to: 1 m 0.05 × 3 · 108 m/s ≈ 67 ns The TOF of gammas scattered from the target and flying with the speed of light c w… view at source ↗
Figure 3.3
Figure 3.3. Figure 3.3: Neutron detector with two PMT’s attached to both each end. Neutron n hits the [PITH_FULL_IMAGE:figures/full_fig_p014_3_3.png] view at source ↗
Figure 3.4
Figure 3.4. Figure 3.4: TOF measurements set-up. The triple coincidence between detectors 1, 2 and 3 from the cosmic ray was used as a start signal to measure the time as a function of distance. the neutron energy by converting the TOF to the neutron velocity. So only one acquisition system channel will be needed in the last case. Some preliminary TOF measurements with 1 PMT attached to the end of the detector were performed an… view at source ↗
Figure 3.5
Figure 3.5. Figure 3.5: TOF measurements with 1 PMT attached to the end of detector. [PITH_FULL_IMAGE:figures/full_fig_p015_3_5.png] view at source ↗
Figure 4.1
Figure 4.1. Figure 4.1: The energy distribution of sum of kinetic energy of two neutrons a and b emitted by fully accelerated fission fragments as seen in laboratory frame [PITH_FULL_IMAGE:figures/full_fig_p017_4_1.png] view at source ↗
Figure 4.3
Figure 4.3. Figure 4.3: Calculated 2n asym￾metry (antiparallel/parallel) as a function of the sum of two neu￾tron energies sum of two neutron energies are represented in [PITH_FULL_IMAGE:figures/full_fig_p018_4_3.png] view at source ↗
Figure 4.5
Figure 4.5. Figure 4.5: 235U photofission cross section taken from ENDF/B-VII.0 Not all photons calculated above will hit the target. Some of them will be lost due to collimation. Assuming the collimation factor is about 50%, the number of photons hitting the target becomes: 4 γt s Nγ = NγI × 50% = 6.25 · 10 (4.7) pulse Wewant one fission per pulse. That can be found by adjusting the target thickness from the equation below: 1 … view at source ↗
Figure 4.6
Figure 4.6. Figure 4.6: Bremsstrahlung spectrum of photons produced by 7 MeV electrons hit￾ting the Al radiator line, like the thickness of the radiator or the collimation hole, can be varied as well. After a reasonable judgment about the beam line elements is done, we still have the possibility to adjust the count rates by varying the LINAC beam parameters, such as the electron pulse width and the electron peak current. 4.3 Be… view at source ↗
Figure 4.7
Figure 4.7. Figure 4.7: Two detector geometry located 2 m away from target [PITH_FULL_IMAGE:figures/full_fig_p024_4_7.png] view at source ↗
read the original abstract

It is well known that two fission fragments (FF's) are emitted essentially back to back in the laboratory frame. That can be used widely in many applications as a unique signature of fissionable materials. However, such fission fragments are difficult to detect. The energy and angular distributions of neutrons, on the other hand, are easy to measure, and that distribution will carry information about the fission fragment's energy and angular spectra, as well as the neutron spectra in the fission fragment rest frame. We propose to investigate the two neutron correlation yield resulting from two FF's as a function of different targets, the angle between the two neutrons and the neutron energies. The preliminary calculation of the two neutron correlation shows a huge asymmetry effect: many more neutrons are emitted anti-parallel to each other than parallel to each other. That asymmetry becomes even more if the energy cut on each neutron is done. This study will potentially permit a new technique for actinide detection for homeland security and safeguards applications as well as improve our knowledge of correlated neutron emission.

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

2 major / 0 minor

Summary. The manuscript proposes a study of two-neutron angular correlations in photofission of actinides, motivated by the back-to-back emission of fission fragments. It claims that a preliminary calculation already demonstrates a large asymmetry favoring anti-parallel neutron emission over parallel emission, with the asymmetry increasing when energy cuts are applied to the neutrons. The work aims to develop this correlation as a detection signature for actinides in homeland security applications and to improve understanding of correlated neutron emission.

Significance. If the reported asymmetry proves robust under realistic photofission kinematics, the proposed neutron-correlation technique could supply a measurable alternative to fragment detection for actinide identification. The manuscript, however, presents no equations, input distributions, or validation steps for the calculation, so the potential impact cannot be assessed from the current text.

major comments (2)
  1. [Abstract] Abstract: The central claim that 'the preliminary calculation of the two neutron correlation shows a huge asymmetry effect' is unsupported by any equations, fragment velocity distributions, neutron multiplicity model, center-of-mass angular distribution, pre-scission component, or integration procedure. This omission is load-bearing because the entire proposed technique rests on the existence and magnitude of the asymmetry.
  2. [Abstract] Abstract: The assertion that the asymmetry 'becomes even more if the energy cut on each neutron is done' is stated without showing how the boost from the fragment rest frame to the lab frame, combined with the chosen energy thresholds, produces this enhancement. No test against known fission data or alternative models is provided.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments on our proposal. The manuscript is a concise outline of the idea and preliminary result; we agree the supporting details are absent and will expand the text to address the concerns.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim that 'the preliminary calculation of the two neutron correlation shows a huge asymmetry effect' is unsupported by any equations, fragment velocity distributions, neutron multiplicity model, center-of-mass angular distribution, pre-scission component, or integration procedure. This omission is load-bearing because the entire proposed technique rests on the existence and magnitude of the asymmetry.

    Authors: We agree the current text provides no supporting details for the calculation. The preliminary result was obtained by assuming back-to-back fission fragments with velocities drawn from literature distributions for actinides, isotropic neutron emission in the fragment rest frame using a Maxwellian spectrum, and Lorentz boosting to the lab frame before integrating over angles and energies. We will add an explicit description of the model, the boost equations, the assumed multiplicity and angular distributions, and the numerical integration procedure in the revised manuscript. revision: yes

  2. Referee: [Abstract] Abstract: The assertion that the asymmetry 'becomes even more if the energy cut on each neutron is done' is stated without showing how the boost from the fragment rest frame to the lab frame, combined with the chosen energy thresholds, produces this enhancement. No test against known fission data or alternative models is provided.

    Authors: The enhancement arises because lab-frame energy cuts preferentially select neutrons emitted forward in the fragment rest frame, which are kinematically aligned with the fragment velocity vector and therefore strengthen the anti-parallel correlation. We acknowledge that neither the kinematic derivation nor any comparison to existing fission data is shown. In revision we will include a quantitative demonstration of the effect with and without cuts together with references to measured neutron spectra for validation. revision: yes

Circularity Check

0 steps flagged

No derivation or equations presented; claim rests on unspecified preliminary calculation.

full rationale

The manuscript proposes a study of two-neutron correlations in photofission but contains no equations, models, integration procedures, or derivation steps. The sole quantitative claim ('preliminary calculation ... shows a huge asymmetry effect') is stated without supporting formalism, fitted parameters, or self-citations that could reduce to inputs by construction. Because no load-bearing derivation chain exists in the text, the circularity criteria cannot be triggered and the score is 0.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The proposal rests on the standard kinematic assumption that fission fragments are emitted back-to-back and on the unstated modeling choices used in the preliminary neutron-correlation calculation; no free parameters or new entities are explicitly introduced in the abstract.

axioms (1)
  • domain assumption Fission fragments are emitted essentially back to back in the laboratory frame.
    Explicitly stated in the first sentence of the abstract as well known.

pith-pipeline@v0.9.0 · 5700 in / 1178 out tokens · 23458 ms · 2026-05-24T22:20:23.919401+00:00 · methodology

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Reference graph

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