{"id":"d6706af8-e1e2-4d62-9b4c-0742a17e610a","arxiv_id":"2606.09656","paper_version":1,"verdict":"UNVERDICTED","confidence":"LOW","novelty_score":5.0,"correctness_risk":"unknown","formal_verification":"none","parameter_count":1,"one_line_summary":"TDDFT calculations of scission neutrons in three fission reactions demonstrate their contribution to high-energy prompt fission neutrons, improving agreement with measured spectra when added to an evaporation model.","lead":"This paper uses time-dependent density functional theory simulations with a large domain to calculate angular and energy distributions of scission neutrons emitted during fission of uranium-235, plutonium-239, and californium-252. It shows these neutrons fill in the high-energy part of the prompt fission neutron spectrum that standard evaporation models underpredict.","discovery_kind":"new_application","skeptic_critique":{"model":"grok-4.3","headline":"Maxwellian evaporation model fitted only to low-energy data may not accurately represent the evaporated component at high energies, allowing potential adjustments to absorb the apparent scission signature.","rationale":"The reader's weakest assumption precisely isolates the comparative subtraction step that converts the TDDFT result into evidence for scission neutrons. Because the argument is differential (evaporation-only fails; evaporation+scission succeeds), uncertainty in the evaporation model directly threatens the claim. No stronger internal inconsistency appears from the provided abstract and claim description; the TDDFT domain-size improvement is noted but does not alter the model-dependence issue.","tokens_in":1790,"tokens_out":371,"duration_ms":14768,"concrete_test":"Refit the Maxwellian parameters to the low-energy experimental PFNS for ^{239}Pu(n_th,f) and ^{252}Cf(sf) while allowing a small high-energy correction term (e.g., a multiplicative factor or additive pre-equilibrium component) constrained only by the same low-energy data; recompute the high-energy residual and check whether the TDDFT scission spectrum is still required to match experiment within uncertainties.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central claim rests on the evaporation-only Maxwellian (constrained solely by low-energy PFNS data) systematically underestimating the measured high-energy yield, with the TDDFT scission spectrum then restoring agreement. This inference is load-bearing because any mismatch between the true high-energy evaporation tail and the extrapolated Maxwellian form (e.g., due to unaccounted pre-equilibrium emission, level-density effects, or fit-parameter freedom) could be absorbed by modest adjustments to the evaporation model itself, eliminating the need to invoke a distinct scission component. The abstract and claim provide no independent validation that the Maxwellian remains valid beyond the fitted low-energy regime.","agreement_with_reader":"agree"},"referee_report":{"model":"grok-4.3","summary":"The manuscript uses time-dependent density functional theory (TDDFT) with an enlarged simulation domain to compute angular and kinetic distributions of scission neutrons emitted in 235U(n_th,f), 239Pu(n_th,f), and 252Cf(sf). Scission neutrons are found to be absent below a threshold of roughly 1.5–2 MeV and to contribute mainly at higher energies. The central result is obtained by superposing the TDDFT scission spectrum onto a Maxwellian evaporation component whose parameters are fixed exclusively by low-energy prompt-fission-neutron-spectrum (PFNS) data; this combined spectrum reproduces the measured high-energy PFNS for 239Pu and 252Cf, whereas the evaporation-only Maxwellian systematically underestimates the data. The authors interpret the improvement as direct evidence for a non-negligible scission-neutron component already present in existing high-energy PFNS measurements.","tokens_in":1971,"tokens_out":642,"duration_ms":18850,"significance":"If the comparison to experiment is robust, the work supplies a microscopic, parameter-constrained link between TDDFT scission-neutron spectra and existing high-energy PFNS data, thereby identifying a possible experimental signature without new measurements. The adoption of a substantially larger computational domain than earlier TDDFT studies is a clear technical improvement. The significance is reduced, however, by the absence of any independent validation that the Maxwellian form remains accurate when extrapolated beyond the fitted low-energy regime.","major_comments":[{"comment":"Abstract and the PFNS-comparison section: the claim that the evaporation-only Maxwellian (parameters fixed solely by low-energy data) systematically underestimates the high-energy yield, while addition of the TDDFT scission spectrum restores agreement, is load-bearing. No test is shown that modest re-adjustment of the Maxwellian parameters or inclusion of other high-energy evaporation mechanisms (pre-equilibrium, level-density variations) could not absorb the same discrepancy, leaving the necessity of a distinct scission component unproven.","section":"Abstract / PFNS comparison"},{"comment":"Methods / results on spectrum extraction: the manuscript provides no quantitative assessment of numerical convergence with respect to simulation-domain size, time-step, or basis truncation, nor any error bands on the extracted scission-neutron spectra that enter the final comparison. These omissions directly affect the reliability of the high-energy tail that is asserted to resolve the experimental mismatch.","section":"Methods and results sections on spectrum extraction"}],"minor_comments":[{"comment":"Abstract: the statement that scission neutrons are “absent below a threshold energy of roughly 1.5–2 MeV” would benefit from an explicit definition of the angular acceptance window used to extract this threshold.","section":"Abstract"},{"comment":"Figure captions and text: several statements refer to “the measured high-energy prompt-fission-neutron yield” without citing the specific experimental data sets or their energy ranges.","section":"Results / figures"}],"recommendation":"major_revision","confidential_remarks":null},"author_rebuttal":{"model":"grok-4.3","summary":"We thank the referee for the careful and constructive review. We respond point-by-point to the major comments below.","responses":[{"response":"The Maxwellian parameters are deliberately fixed only by low-energy PFNS data to prevent circular fitting to the high-energy region under discussion. This follows standard practice in the field. The systematic under-prediction at high energies is a documented limitation of pure evaporation models. Adding the TDDFT scission spectrum (with no adjustment to the evaporation component) accounts for the observed excess. While other mechanisms could in principle contribute, the microscopic, parameter-free TDDFT result provides a specific prediction that matches the discrepancy. We will revise the manuscript to explicitly note that parameters remain unchanged and to discuss why re-adjustment would violate the low-energy constraint.","revision_made":"partial","referee_comment":"[Abstract / PFNS comparison] Abstract and the PFNS-comparison section: the claim that the evaporation-only Maxwellian (parameters fixed solely by low-energy data) systematically underestimates the high-energy yield, while addition of the TDDFT scission spectrum restores agreement, is load-bearing. No test is shown that modest re-adjustment of the Maxwellian parameters or inclusion of other high-energy evaporation mechanisms (pre-equilibrium, level-density variations) could not absorb the same discrepancy, leaving the necessity of a distinct scission component unproven."},{"response":"We agree that explicit convergence tests and uncertainty estimates would strengthen the results. Although the enlarged domain is presented as an improvement, quantitative assessments were omitted. In the revised manuscript we will add convergence studies varying domain size and time step, together with estimated error bands on the extracted spectra obtained from these variations.","revision_made":"yes","referee_comment":"[Methods and results sections on spectrum extraction] Methods / results on spectrum extraction: the manuscript provides no quantitative assessment of numerical convergence with respect to simulation-domain size, time-step, or basis truncation, nor any error bands on the extracted scission-neutron spectra that enter the final comparison. These omissions directly affect the reliability of the high-energy tail that is asserted to resolve the experimental mismatch."}],"tokens_in":1529,"tokens_out":458,"duration_ms":24089,"standing_objections":[]},"desk_editor":{"model":"grok-4.3","letter":"The central point is that larger-domain TDDFT runs produce scission neutrons that start above roughly 1.5-2 MeV and, when added to a Maxwellian fitted only at low energies, bring the calculated prompt fission neutron spectrum into line with data for 239Pu and 252Cf.\n\nThe calculation itself extends earlier TDDFT fission work by using a bigger simulation volume and pulling out angular and kinetic distributions at angles where scission neutrons dominate the high-energy region. That supplies a microscopic source for the excess that evaporation-only models miss.\n\nThe paper does a straightforward job of showing the TDDFT spectrum has the right shape to fix the mismatch once the evaporation piece is fixed at low energy.\n\nThe soft spot is the evaporation model. It is constrained only below a few MeV and then assumed to continue as a Maxwellian at higher energies. Any real deviation in the evaporated tail from pre-equilibrium emission or level-density effects could be absorbed by modest changes to the fit parameters, removing the need for a separate scission component. The abstract supplies no convergence checks, error estimates, or independent validation of that extrapolation, so the evidence remains suggestive rather than conclusive.\n\nThis is aimed at fission modelers who need better high-energy neutron yields for reactor and safeguards applications. Readers working on microscopic fission dynamics will find the extracted distributions useful; those focused on data libraries will want to see the evaporation assumption tested more directly.\n\nIt deserves peer review because the TDDFT part is a concrete advance over prior runs in the same framework, even though the interpretation of the high-energy match needs careful checking on the evaporation side.","headline":"TDDFT scission neutrons fill the high-energy PFNS gap only if the low-energy Maxwellian evaporation model extrapolates accurately, which the abstract does not verify.","tokens_in":2452,"tokens_out":405,"would_cite":false,"duration_ms":13814,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"grok-4.3","headline":"Scission neutrons explain the excess high-energy yield in prompt fission neutron spectra for plutonium and californium when added to evaporation models.","keywords":["scission neutrons","prompt fission neutrons","time-dependent density functional theory","nuclear fission","neutron energy spectrum","evaporation model","angular distribution","fission dynamics"],"falsifier":"A high-precision measurement of the prompt neutron spectrum in 239Pu(n_th,f) that shows the high-energy tail is fully reproduced by an evaporation-only model without any additional high-energy component would falsify the claim.","tokens_in":2703,"feed_emoji":"","tokens_out":820,"duration_ms":20761,"temperature":0.7,"pith_summary":"The paper uses time-dependent density functional theory to model scission-neutron emission during fission in three systems: thermal-neutron fission of uranium-235 and plutonium-239, plus spontaneous fission of californium-252. With a larger simulation domain, it extracts angular and energy distributions showing that scission neutrons are absent below roughly 1.5 to 2 MeV at selected angles and instead populate the higher-energy region of the prompt spectrum. Adding the calculated scission spectrum to a Maxwellian evaporation model fitted only to low-energy data reproduces the measured high-energy yields for plutonium and californium, while an evaporation-only model underestimates them. This match supplies evidence that a non-negligible scission-neutron component is already visible in existing experimental spectra.","feed_headline":"Scission neutrons fill high-energy gap in fission spectra","feed_subtitle":"TDDFT results plus evaporation model match measured yields for Pu-239 and Cf-252 where evaporation alone underpredicts the tail.","key_machinery":"Time-dependent density functional theory simulations that compute the angular and kinetic distributions of scission neutrons emitted during the fission process.","core_discovery":"Scission-neutron emission is investigated in 235U(n_th,f), 239Pu(n_th,f) and 252Cf(sf) within time-dependent density functional theory. Using a substantially larger simulation domain than in previous studies, the angular and energy distributions of emitted scission neutrons are extracted over a specific range of emission angles. At these angles, scission neutrons are absent below a threshold energy of roughly 1.5--2 MeV, and instead contribute predominantly to the higher energy part of the prompt fission neutron spectrum. Combining the calculated scission-neutron spectrum with a Maxwellian model for the evaporated component, constrained by low-energy experimental data, reproduces the measure","pith_inferences":["Similar TDDFT calculations applied to other fissioning nuclei could map how the scission contribution varies with mass and excitation energy.","Angular-resolved neutron measurements at facilities could directly test the predicted 1.5--2 MeV threshold for scission neutrons.","Reactor simulations that rely on accurate high-energy neutron spectra may need to incorporate an explicit scission component derived from microscopic dynamics."],"forward_implications":["The high-energy prompt-fission-neutron yield in 239Pu(n_th,f) and 252Cf(sf) is reproduced only when the scission-neutron spectrum is included.","Evaporation-only models systematically underestimate the observed high-energy prompt neutron yields.","A non-negligible scission-neutron component is required to explain existing high-energy prompt fission neutron spectra.","The angular and energy distributions extracted from the simulations identify a clear energy threshold below which scission neutrons do not appear at the studied angles."],"fun_headline_variants":["TDDFT shows scission neutrons above 1.5 MeV in fission","Scission neutrons absent below 1.5 MeV per TDDFT","TDDFT scission spectrum matches high-energy fission yields","Scission neutrons explain high-energy yields in fission"],"cache_read_input_tokens":2112,"weakest_assumption_plain":"A Maxwellian evaporation model fitted only to low-energy data continues to describe the evaporated neutron component accurately across the full energy range without further adjustments.","fun_headline_variants_meta":{"raw":{"variants":["TDDFT shows scission neutrons above 1.5 MeV in fission","Scission neutrons absent below 1.5 MeV per TDDFT","TDDFT scission spectrum matches high-energy fission yields","Scission neutrons explain high-energy yields in fission"]},"model":"grok-4.3","cost_usd":0.007561,"raw_usage":{"total_tokens":3503,"prompt_tokens":742,"num_sources_used":0,"completion_tokens":69,"cost_in_usd_ticks":75612000,"prompt_tokens_details":{"text_tokens":742,"audio_tokens":0,"image_tokens":0,"cached_tokens":256},"completion_tokens_details":{"audio_tokens":0,"reasoning_tokens":2692,"accepted_prediction_tokens":0,"rejected_prediction_tokens":0}},"tokens_in":742,"tokens_out":69,"duration_ms":16306,"temperature":1.0,"reasoning_tokens":2692,"cache_read_input_tokens":256,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-06-27T14:46:40.706408+00:00","model_set":{"reader":"grok-4.3"},"falsifier":"A high-precision measurement of the prompt neutron spectrum in 239Pu(n_th,f) that shows the high-energy tail is fully reproduced by an evaporation-only model without any additional high-energy component would falsify the claim.","supporting_citations":[],"review_version":1}