In Situ Photoneutron Calibration

• In situ photoneutron calibration is the use of gamma-emitting radioisotopes with beryllium converters to generate low-energy neutrons directly within the detector environment.
• The method employs specific reactions (e.g., 9Be(γ,n)8Be) and source classes like 124Sb-Be and 88Y-Be to produce quasi-monoenergetic neutrons critical for calibrating dark-matter and neutrino detectors.
• Robust shielding, detailed transport simulation, and tailored deployment geometries are key to accurately measuring nuclear recoils and validating detector response models.

In situ photoneutron calibration is the use of a deployed $\gamma$-emitting radioisotope and beryllium converter to generate low-energy neutrons inside, or immediately adjacent to, the final detector configuration, so that nuclear-recoil response is measured under operating geometry, shielding, and background conditions. In the implementations described for dark-matter and coherent elastic neutrino-nucleus scattering experiments, the underlying reaction is typically ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$, driven by sources such as ${}^{124}$Sb or ${}^{88}$Y, and the calibration target is the detector medium itself or a well-defined external beamline coupled to a backing detector (Biekert et al., 2023). The technique is used to probe sub-keV to few-keV nuclear recoils, to extract quenching factors or light and charge yields, and to validate transport and response models in the experiment’s installed configuration (Collaboration et al., 2024).

1. Nuclear basis and source classes

The central photodisintegration process is

$${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$

In the SBC calibration plan, the neutron binding energy in ${}^{9}$Be is given as $Q_{\rm Be}=1.665$ MeV, while the XENONnT and Y-88/Be descriptions quote $Q_{\rm th}=1.667$ MeV or $S_n=1.667$ MeV [(collaboration, 25 Nov 2025); (Collaboration et al., 2024); (Collar, 2013)]. In the center-of-mass frame the neutrons are essentially monoenergetic; in the laboratory frame there is a small Doppler and recoil broadening, and the SBC plan states that simulated line shapes have $\sim10\%$ width (collaboration, 25 Nov 2025).

Two source classes dominate the reported implementations. The first is ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$0Sb-Be. For the portable source, the relevant ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$1 lines are ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$2 keV with ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$3 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$4 mb, yielding dominant ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$5 keV neutrons, and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$6 keV with ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$7 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$8 mb, yielding sub-dominant ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$9 keV neutrons; the normalized neutron-production rate per ${}^{124}$0Sb decay is ${}^{124}$1 n/decay for the ${}^{124}$2 keV branch (Biekert et al., 2023). The second is ${}^{124}$3Y-Be. In XENONnT, ${}^{124}$4 MeV ${}^{124}$5 rays from ${}^{124}$6Y decay produce quasi-monoenergetic ${}^{124}$7 keV neutrons on ${}^{124}$8Be, with ${}^{124}$9 mb and a Monte Carlo photodisintegration probability per ${}^{88}$0 of ${}^{88}$1 (Collaboration et al., 2024). In LZ, the same source also includes the ${}^{88}$2 MeV ${}^{88}$3 line, so the source produces both ${}^{88}$4 keV and ${}^{88}$5 keV neutrons, with ${}^{88}$6 mb and ${}^{88}$7 mb, respectively (Aalbers et al., 18 Sep 2025). Collar’s earlier Y-88/Be study frames the same source class as a compact method for generating a clean, quasi-monochromatic ${}^{88}$8 keV neutron field for low-energy recoil measurements in NaI[Tl], bubble chambers, and noble liquids (Collar, 2013).

The generic neutron-yield expression used in the SBC plan is

${}^{88}$9

with the monoenergetic approximation

$${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$0

This formalism emphasizes that in situ photoneutron calibration is fundamentally a source-strength, cross-section, and geometry problem rather than a detector-only problem (collaboration, 25 Nov 2025).

2. Shielding, transport, and deployment geometry

A defining feature of in situ photoneutron calibration is that the source is integrated into the installed apparatus rather than into a separate beam test. The portable $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$1Sb-$${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$2Be source realizes this explicitly as a transportable neutron beam system. Its central iron filter rod has diameter $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$3 cm and length $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$4 cm, and the source assembly includes successive layers of W, Pb, stainless steel, and Al in a T-slot frame, together with a detachable $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$5 cm-thick borated-PE sleeve and a circular beam aperture of approximately $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$6 cm aligned to the filter axis (Biekert et al., 2023). The design relies on the coincidence between the $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$7 keV neutron energy and the low interaction cross-section with iron: for $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$8 keV, $${}^{9}{\rm Be} + \gamma \;\longrightarrow\; {}^{8}{\rm Be} + n .$$9 barn gives a neutron mean free path of ${}^{9}$0 cm in iron, while for ${}^{9}$1 MeV ${}^{9}$2 rays in iron, ${}^{9}$3 cm${}^{9}$4 implies attenuation of approximately ${}^{9}$5 over ${}^{9}$6 cm (Biekert et al., 2023). The resulting system has total shield mass ${}^{9}$7 kg, fits in a ${}^{9}$8 gal Type-A drum for road transport, and is described as suitable for existing beam ports or water-filled calibration pits (Biekert et al., 2023).

The XENONnT deployment uses a different geometry but the same in situ principle. The ${}^{9}$9Y disc, with activity $Q_{\rm Be}=1.665$0 kBq, is contained in a stainless-steel capsule and sandwiched between two $Q_{\rm Be}=1.665$1 mm $Q_{\rm Be}=1.665$2 $Q_{\rm Be}=1.665$3 mm Be cylinders inside a tungsten shielding box of outer dimensions $Q_{\rm Be}=1.665$4 cm $Q_{\rm Be}=1.665$5 $Q_{\rm Be}=1.665$6 cm $Q_{\rm Be}=1.665$7 $Q_{\rm Be}=1.665$8 cm (Collaboration et al., 2024). The assembly is lowered until the front face of the tungsten box is aligned with the TPC mid-cathode plane, approximately $Q_{\rm Be}=1.665$9 cm from the cryostat wall, and an air-filled stainless-steel water-displacement box fills the gap to reduce neutron energy degradation (Collaboration et al., 2024).

LZ likewise deploys a stacked YBe source inside a $Q_{\rm th}=1.667$0 cm $Q_{\rm th}=1.667$1 $Q_{\rm th}=1.667$2 cm tungsten block, lowered through a dedicated cut-out in the top acrylic Outer Detector into the water tank, where it rests above the outer cryostat with the $Q_{\rm th}=1.667$3Y located $Q_{\rm th}=1.667$4 cm above the TPC gate electrode (Aalbers et al., 18 Sep 2025). The description stresses that the calibration is fully in situ: no disassembly of the TPC is required, and source exchange is done by removing the Gd-doped liquid-scintillator plug used in normal running and inserting the YBe assembly (Aalbers et al., 18 Sep 2025).

The SBC plan makes explicit that in situ deployment need not imply collimation. There, the source-plus-Be assembly is lowered inside a $Q_{\rm th}=1.667$5 cm ID stainless-steel calibration tube whose bottom sits $Q_{\rm th}=1.667$6 cm above the top of the argon, and a neutron traverses $Q_{\rm th}=1.667$7 cm stainless-steel source tube cap, $Q_{\rm th}=1.667$8 cm stainless-steel pressure vessel wall, $Q_{\rm th}=1.667$9 cm liquid CF$S_n=1.667$0 hydraulic fluid, and $S_n=1.667$1 cm fused-silica outer vessel before reaching the argon (collaboration, 25 Nov 2025). No additional moderation or collimation is used; the tube geometry defines the angular acceptance, and scattering in CF$S_n=1.667$2/silica slightly softens the spectrum before it reaches the argon (collaboration, 25 Nov 2025).

3. Recoil generation and detector observables

The calibration observable is the detector response to elastic neutron scattering. The SBC plan gives the recoil-energy relation for a neutron of energy $S_n=1.667$3 scattering on $S_n=1.667$4Ar at laboratory angle $S_n=1.667$5 as

$S_n=1.667$6

with the recoil spectrum obtained from

$S_n=1.667$7

This formalism makes clear that in situ photoneutron calibration links source transport to the recoil spectrum through the neutron flux spectrum at the active medium (collaboration, 25 Nov 2025).

For LXe TPCs, the observables are the prompt scintillation signal $S_n=1.667$8 and the delayed proportional scintillation signal $S_n=1.667$9. In LZ, $\sim10\%$0 is collected by $\sim10\%$1 two-inch-diameter Hamamatsu R11410 PMTs, while $\sim10\%$2 is produced by electrons drifted approximately $\sim10\%$3 cm at $\sim10\%$4 V/cm from liquid into gas and extracted with an extraction field of approximately $\sim10\%$5 kV/cm; the instrumental gains in the YBe setting are $\sim10\%$6 phd/photon and $\sim10\%$7 phd/electron (Aalbers et al., 18 Sep 2025). XENONnT similarly uses corrected $\sim10\%$8 and $\sim10\%$9 distributions, with an analysis region requiring ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$00 observed photons and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$01 PE (Collaboration et al., 2024).

For bubble chambers, the observable is nucleation rather than a continuous energy estimator. In the SBC plan, three cameras provide stereo reconstruction, eight piezoelectric transducers record the time and three-dimensional position of each bubble, and a pressure transducer sees the rapid drop in pressure when a bubble forms (collaboration, 25 Nov 2025). The scintillation channel, using ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$02 SiPMs sensing ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$03 nm LAr scintillation with wavelength shifted by ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$04 ppm xenon doping, is used only to veto high-energy backgrounds or to tag neutron captures, not for bubble timing (collaboration, 25 Nov 2025). Each expansion cycle superheats the top ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$05 kg of LAr, the chamber recompresses after bubble formation with approximately ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$06 s dead time, and no direct energy measurement is made in each bubble event; instead bubble counts are compared with simulation-predicted recoil spectra (collaboration, 25 Nov 2025). This is a salient methodological distinction: in situ photoneutron calibration does not always mean event-by-event recoil-energy reconstruction.

The older NaI[Tl] implementation uses yet another observable, the quenching factor ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$07, extracted by subtracting a gamma-only configuration from a neutron-plus-gamma configuration and fitting the residual spectrum with MCNP-PoliMi (Collar, 2013). That approach shows that photoneutron calibration can serve both threshold-setting and response-function measurement.

4. Measurement protocols, simulation, and inference

In situ photoneutron calibration typically combines source characterization, detailed transport simulation, and detector-response inference. The portable ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$08Sb-${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$09Be source was characterized with a hydrogen-gas proportional counter and a NaI(Tl) detector (Biekert et al., 2023). The HGPC used an LND 27044 sphere with diameter ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$10 cm, H${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$11 at ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$12 bar and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$13C, an energy threshold of approximately ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$14 keV, resolution ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$15, and neutron/electron pulse-shape discrimination via ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$16–${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$17 rise time with neutron selection ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$18s; gamma background was measured by inserting a borated-PE plug and subtracted bin by bin (Biekert et al., 2023). The NaI(Tl) detector was a ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$19 cm ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$20 ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$21 cm cylinder calibrated with ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$22Am, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$23Co, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$24Ba, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$25Cs, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$26Na, and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$27Th, and the full-energy peaks at ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$28 keV and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$29 keV matched simulation within ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$30 (Biekert et al., 2023).

The SBC plan formalizes the inference problem in terms of simulated recoil spectra ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$31 and a nucleation-efficiency function ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$32 parameterized piecewise with five nodes at efficiencies ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$33, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$34, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$35, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$36, and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$37 (collaboration, 25 Nov 2025). For each source ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$38, the expected bubble count is written as

${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$39

and the total event count for photoneutrons as

${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$40

A global ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$41 or MCMC is then used to fit the five node energies and nuisance parameters for source strengths and simulation uncertainties (collaboration, 25 Nov 2025).

XENONnT adopts a forward model in Appletree, combining the NEST v2 yield model with a detector-response model, and fits the corrected ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$42 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$43 distributions with a binned Poisson likelihood

${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$44

Twenty-one parameters are sampled via an affine-invariant MCMC using emcee, and the accidental-coincidence spectrum derived from the ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$45Y-PVC gamma-only run is included with a floating normalization constrained by a Gaussian prior of ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$46 (Collaboration et al., 2024).

LZ uses NEST to convert recoil energy into expected ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$47 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$48, but in its first in situ YBe analysis it restricts the fit to the ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$49 keV recoil population and simultaneously fits the ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$50 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$51 distributions to extract an overall ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$52 scale factor while leaving all other NEST parameters fixed (Aalbers et al., 18 Sep 2025). Backgrounds from the ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$53 keV neutron mode and accidentals are included as fixed-shape components, and delayed Outer Detector tagging confirms neutron capture with an ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$54 veto tag efficiency (Aalbers et al., 18 Sep 2025).

Across these implementations, Geant4 appears as the standard neutron-transport engine. The portable source uses the Geant4 Shielding physics list for beam-flux prediction (Biekert et al., 2023), while the SBC plan calls for a Geant4 model of the full geometry with ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$55–${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$56 neutrons per source (collaboration, 25 Nov 2025). A plausible implication is that in situ photoneutron calibration is as much a validation of the transport model as of the detector response model.

5. Representative implementations and quantitative outcomes

The portable ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$57Sb-${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$58Be source is designed as a dedicated low-energy recoil calibrator. For a ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$59 GBq ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$60Sb source, Geant4 predicts ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$61 n/cm${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$62/s at the beam exit, of which ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$63 lies in ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$64–${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$65 keV, giving ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$66–${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$67 n/cm${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$68/s; the measurement using the HGPC-tagged recoil endpoint yields ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$69–${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$70 n/cm${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$71/s (Biekert et al., 2023). The corresponding gamma flux above ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$72 keV is ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$73 ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$74/cm${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$75/s, the neutron-to-gamma ratio at the exit is approximately ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$76, and contamination from ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$77 keV neutrons is stated to be ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$78 of the total (Biekert et al., 2023). The same work reports that an Eljen-301 liquid scintillator backing detector is sensitive to incident neutrons from ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$79 to ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$80 keV, with tagging efficiency rising from approximately ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$81 at ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$82 keV to approximately ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$83 at ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$84 keV, and gives an expected recoil precision at the ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$85 eV${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$86 scale of ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$87 per event in a typical dark-matter or CEnuNS detector with ring-tagging geometry (Biekert et al., 2023).

XENONnT used a ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$88YBe source emitting ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$89 keV neutrons to calibrate liquid-xenon light and charge yields for the first time in situ (Collaboration et al., 2024). After data selection, ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$90 events were accumulated from ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$91 hours of exposure, while the expected background was ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$92 accidental-coincidence events estimated from a dedicated ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$93 hour background calibration run with a Yttrium-PVC gamma-only source and data-driven modeling (Collaboration et al., 2024). The analysis extracted ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$94 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$95 between ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$96 keV${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$97 and ${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$98 keV${}^{9}{\rm Be}(\gamma,n){}^{8}{\rm Be}$99 at ${}^{124}$00 V/cm, with the lowest observable energy set by ${}^{124}$01 to ${}^{124}$02 keV${}^{124}$03 (Collaboration et al., 2024).

LZ reports its first in situ photoneutron calibration using a custom ${}^{124}$04Y-${}^{124}$05Be source (Aalbers et al., 18 Sep 2025). For the initial ${}^{124}$06 MBq source, the absolute neutron rates at production are ${}^{124}$07 n/s and ${}^{124}$08 n/s (Aalbers et al., 18 Sep 2025). Elastic scattering of the ${}^{124}$09 keV component gives a maximal recoil energy of ${}^{124}$10 keV${}^{124}$11 in xenon, and the unfolded single-scatter spectrum from this component is described as quasi-flat up to ${}^{124}$12 keV${}^{124}$13 with mean ${}^{124}$14 keV${}^{124}$15 (Aalbers et al., 18 Sep 2025). The best-fit neutron rate at the source is ${}^{124}$16 n/s, in excellent agreement with the analytic ${}^{124}$17 n/s; the best-fit ${}^{124}$18 and ${}^{124}$19 spectra match simulations with ${}^{124}$20-values of ${}^{124}$21 and ${}^{124}$22; and the measured ${}^{124}$23 at ${}^{124}$24 keV${}^{124}$25 is ${}^{124}$26 below the default NEST central value but within its ${}^{124}$27 band (Aalbers et al., 18 Sep 2025).

The SBC Collaboration’s calibration plan extends the method to a ${}^{124}$28 kg liquid-argon bubble chamber with ${}^{124}$29 eV target threshold (collaboration, 25 Nov 2025). It proposes using, for example, ${}^{124}$30Ci ${}^{124}$31Bi-Be and ${}^{124}$32Ci ${}^{124}$33Sb-Be sources to give approximately ${}^{124}$34 bubbles/hour above threshold, with ${}^{124}$35 h of live data at fixed thermodynamic conditions for each source (collaboration, 25 Nov 2025). Under standard systematics, the final threshold uncertainty inferred from the width of the ${}^{124}$36 pull distribution over ${}^{124}$37 mock datasets is ${}^{124}$38, and under improved systematics it is ${}^{124}$39 (collaboration, 25 Nov 2025).

The earlier Y-88/Be study by Collar demonstrates the same logic in NaI[Tl], reporting a measured sodium quenching factor below ${}^{124}$40 keV${}^{124}$41 that falls from ${}^{124}$42 at ${}^{124}$43 keV${}^{124}$44 to ${}^{124}$45 below ${}^{124}$46 keV${}^{124}$47 (Collar, 2013). The abstract states that this is considerably smaller than the ${}^{124}$48 typically adopted in interpretations of the DAMA/LIBRA dark-matter experiment and results in a marked increase of its tension with other searches under the standard set of phenomenological assumptions (Collar, 2013).

6. Uncertainties, limitations, and points of interpretation

The uncertainty budget in in situ photoneutron calibration is distributed across source strength, photoneutron production, neutron transport, geometry, and detector-response inference. The SBC plan lists source-strength calibration at ${}^{124}$49 near-term and ${}^{124}$50 in future, photoneutron cross-section uncertainty at ${}^{124}$51 background and ${}^{124}$52 goal, Geant4 modeling of neutron transport at ${}^{124}$53–${}^{124}$54 common mode, geometry tolerances giving flux shifts of ${}^{124}$55 for tube-position changes of a few mm, residual rock and muon neutron backgrounds stable to ${}^{124}$56, and fit-model systematics from the choice of piecewise nodes (collaboration, 25 Nov 2025). These are combined via pull terms in the ${}^{124}$57 and by rerunning fits under extreme systematic shifts (collaboration, 25 Nov 2025).

Gamma backgrounds are a recurrent practical limitation rather than a negligible detail. In the portable ${}^{124}$58Sb-Be system, dedicated gamma characterization is mandatory, and the measured beam still contains ${}^{124}$59 gammas/cm${}^{124}$60/s above ${}^{124}$61 keV per ${}^{124}$62 GBq source activity at the exit (Biekert et al., 2023). XENONnT treats accidental-coincidence backgrounds explicitly through a gamma-only ${}^{124}$63Y-PVC run and a data-driven model (Collaboration et al., 2024). Collar’s Y-88/Be description states that after ${}^{124}$64–${}^{124}$65 cm of lead shielding, the gamma-induced electron-recoil rate under the nuclear-recoil search window is less than ${}^{124}$66 of the neutron-induced signal, but emphasizes the need for Be-versus-dummy subtraction and control of laboratory-origin neutrons (Collar, 2013).

A common misconception is that the neutron field remains perfectly monoenergetic once deployed. The source reactions are monoenergetic or quasi-monoenergetic at production, but the flux entering the active volume is often softened or broadened by detector materials. The SBC plan states that neutron scattering in CF${}^{124}$67/silica slightly softens the spectrum before it reaches the argon (collaboration, 25 Nov 2025). LZ reports that the entering-neutron spectrum falls smoothly from ${}^{124}$68 down to zero up to each endpoint because most neutrons are moderated by the Ti cryostat and SS/PTFE structures before entering the active LXe (Aalbers et al., 18 Sep 2025). XENONnT uses an air-filled gap specifically to reduce neutron energy degradation (Collaboration et al., 2024). This suggests that “in situ” is not merely a convenience of placement; it is part of the calibration observable because the installed materials reshape the spectrum.

Another misconception is that in situ photoneutron calibration always yields direct per-event recoil energies. The portable ${}^{124}$69 keV beam paired with a backing detector is expressly designed for neutron scattering calibration experiments with percent-level recoil tagging (Biekert et al., 2023). By contrast, the SBC bubble-chamber calibration counts bubbles and fits ${}^{124}$70 statistically, without direct event-by-event recoil measurement (collaboration, 25 Nov 2025). LXe TPC analyses lie between these extremes, inferring yield models from distributions in corrected scintillation and ionization observables rather than from uniquely determined recoil energies for every event (Collaboration et al., 2024, Aalbers et al., 18 Sep 2025).

Taken together, the reported implementations show a mature calibration modality for low-threshold dark-matter and CEnuNS detectors. They establish that source physics, shielding design, transport simulation, and response inference must be treated as a coupled system, and they show that the principal value of the method is precisely that it measures nuclear-recoil response in the experiment’s operating configuration rather than in a detached proxy setup.