Experiments detecting neutrinos via the (ν_e, e−) reaction on ⁷¹Ga systematically observe lower than expected interaction rates. This unexplained phenomenon, demonstrated most recently by the BEST experiment with a high statistical significance [1], is known as the “gallium anomaly.” The discrepancy is so far unexplained and could be an exciting potential indicator of new neutrino physics [2,3]. However, it has been pointed out that a larger value of the half-life of ⁷¹Ge (12.5 – 13.5 days vs. 11.43 days measured in 1985 [4,5]) used to compute the neutrino cross sections on ⁷¹Ga could reduce or eliminate the anomaly [6] and that new measurements were needed [7]. 

This question is thoroughly addressed in E.B. Norman et al.’s new paper, “Half-life of ⁷¹Ge and the gallium anomaly,” published on May 30th 2024 in Physical Review C [8] as an Editor’s Suggestion. The authors from UC Berkeley, LBNL, and LLNL carried out three separate high-precision spectral measurements with planar HPGe detectors at the LLNL Nuclear Counting Facility: the rate of electron capture decay of ⁷¹Ge to ⁷¹Ga was measured directly, as well as in relation to ⁵⁷Co and ⁵⁵Fe radioactive standards. In the latter technique, fitting the ratio of the ⁷¹Ge activity to that of the well-known standard provides a measure of the difference in their decay rates, while eliminating many systematic effects. Figs. 2 and 3 show the spectra and the decay rate fits, respectively. 

Both germanium oxide and crystalline High Purity germanium samples were utilized, with the latter prepared at LBNL’s Semiconductor Detector Laboratory (SDL) by NSD Research Scientist Alexey Drobizhev. The material was irradiated in the graphite reflector of the McClellan Nuclear Research Center (MNRC) TRIGA reactor at UC Davis to create the relatively short-lived ⁷¹Ge isotope. The samples were counted continuously for over two months at LLNL. NSD’s Yury Kolomensky contributed to the high quality statistical analysis featured in this paper. 

The final result gives a ⁷¹Ge half-life of 11.468 ± 0.008 days—fully consistent with prior measurements (Fig. 1), while significantly more precise. Importantly, it rules out the longer half-life values that would be needed to explain the neutrino event deficit seen by gallium experiments. Thus, the gallium anomaly remains a mystery with potential for more beyond-the-standard model physics in the neutrino sector. 

References
[1] V. V. Barinov et al., Results from the Baksan experiment on sterile transitions (BEST), Phys. Rev. Lett. 128, 232501 (2022) DOI: https://doi.org/10.1103/PhysRevLett.128.232501
[2] J. N. Abdurashitov et al., Measurement of the response of a Ga solar neutrino experiment to neutrinos from a ³⁷Ar source, Phys. Rev. C 73, 045805 (2006) DOI: https://doi.org/10.1103/PhysRevC.73.045805
[3] M. Laveder, Unbound neutrino roadmaps, Nucl. Phys. B 168, 344 (2007) DOI: https://doi.org/10.1016/j.nuclphysbps.2007.02.037
[4] B. Singh and J. Chen, Nuclear structure and decay data for A = 71 isobars, Nucl. Data Sheets 188, 1 (2023) DOI: https://doi.org/10.1016/j.nds.2023.02.001
[5] W. Hampel and L. P. Remsberg, Half-Life of 71Ge, Phys. Rev. C 31, 666 (1985) DOI: https://doi.org/10.1103/PhysRevC.31.666
[6] C. Giunti, Y. F. Li, C. A. Ternes, and Z. Xin, Inspection of the detection cross section dependence of the gallium anomaly, Phys. Lett. B 842, 137983 (2023) DOI: https://doi.org/10.1016/j.physletb.2023.137983
[7]  S. R. Elliott, V. Gavrin, and W. Haxton, The gallium anomaly, arXiv:2306.03299v1 DOI: https://doi.org/10.48550/arXiv.2306.03299
[8] E.B. Norman et al. “Half-life of  ⁷¹Ge  and the gallium anomaly.” Phys. Rev. C 109, 055501 (2024) DOI: https://doi.org/10.1103/PhysRevC.109.055501

Photons are important probes because they couple to all charged particles. The coupling strength is determined by measuring photon splitting into particle-antiparticle pairs.   These strengths have been previously measured for electrons, for quarks, and for one type of meson – the pion.  In a paper published in Physical Review Letters,, the ALICE Collaboration reports on the first measurement of coherent photoproduction of a charged kaon pair (K⁺K^–). The data allows us to compare the behavior of heavier mesons, containing strange quarks (kaons) with lighter mesons composed only of up and down quarks (pions). The study found that the coupling of kaons and pions was comparable, after accounting for the large mass difference.    

The data came from ultra-peripheral collisions (UPCs) of relativistic lead nuclei, produced at CERNs Large Hadron Collider (LHC).  In UPCs, the two nuclei do not physically collide, but interact at long range, via photon exchange.  A photon from one nucleus can fluctuate to a virtual (evanescent) K⁺K^– meson pair (particle and antiparticle) and then one of the kaons can scatter elastically (and coherently) from the other nucleus, emerging as a real K⁺K^– pair.  The final state kaon pairs have a very low transverse momentum (pT), as is seen in the figure.  This showed that the kaon-nucleus scattering was coherent (elastic),  with coupling to all of the protons and neutrons in the target.   The analysts – the Relativistic Nuclear Collision program’s  Minjung Kijm, Spencer Klein and Mateusz Ploskon – applied stringent particle identification cuts to select a pure sample of kaon pairs. 

With that, the ALICE collaboration found that the couplings were similar to those of pions (with only ¼ the mass), after adjustment for the different meson mass, showing that different mesons behave similarly, despite difference in quark content and mass.  Looking ahead, with the upgraded ALICE data acquisition system, during LHC Runs 3 and 4, ALICE should collect more than 100 times more UPC events than were used here, allowing for precision measurements of kaon pairs, and also allowing us to look at pairs of heavier mesons, and even baryon pairs.  A more comprehensive set of results will allow us to study trends based on meson masses and other properties.

Figure 1: The transverse momentum (p_T, left) and daikon mass (right) spectra for the ALICE event sample. The pT spectrum shows that the production is coherent over the entire target. The production rate is above expectations from the ϕ meson alone.

The goats are back at the Lab, so it must be summer. It is also the time to welcome nine summer interns who are working with teams across the Division.  

There was much to celebrate since the last newsletter. It was a pleasure to join, together with several NSD members, the flag raising ceremony to celebrate Pride Month. An important reminder of our commitment to fostering a workplace culture where everyone feels welcome and valued. We also celebrate several NSD members who were recognized for their outstanding work. Congratulations to Wick Haxton, Erich Leistenschneider, Gabriel Orebi Gann, and Xin-Nian Wang on their respective well deserved awards (see Fragment for more details).

On the science side, we can celebrate the breakthrough by our Heavy Element team, who in collaboration with their international partners succeeded to produce two events of Element 116 (livermorium) by bombarding a plutonium target with a titanium-50 beam at the 88-Inch Cyclotron. Based on the measured production rate the search for new Element 120 using titanium beam seems feasible.

In this issue of the NSD Newsletter we are featuring recent physics results from ALICE, led by NSD scientists, that investigate the coherent photoproduction of pairs of kaons in ultra-peripheral collisions (UPCs) of relativistic lead nuclei, which for the first time gave access to the photon-kaon coupling strength. We also learn about a remarkable measurement of the 71Ge half-life, which rules out potential explanation of the so-called “gallium anomaly”, leaving the door open for potentially more physics beyond-the-standard model in the neutrino sector. Another exciting development we learn about, is how machine learning methods, automation, and robotics are being used for nuclear safeguards applications. Finally, the GRETA project has achieved another major milestone by safely completing the challenging move of the main mechanical structures from the 88-Inch high bay to the laboratory ‘K-area’ space in the basement level of Bldg. 88 where the final assembly, systems integration and testing will take place. The move was a real team effort of the LBNL riggers with support of the 88-Inch mechanical crew. Congratulations. The pictures in this issue of the newsletter only tell part of the story.

Overall, it is fantastic to see the progress and celebrate these achievements, the recognition of our people, and the commitment to the lab’s stewardship values and IDEA principles. Wishing everyone an enjoyable summer.

Sincerely,

It is with deep sadness that we note the recent passing of retired NSD scientists Doug Greiner and Joe Cerny.

Several NSD staff members and affiliates attended the April Meeting of the American Physical Society which was held in Sacramento during the first week of April where they delivered presentations on a range of topics.

NSD celebrates several recent promotions.  Heather Crawford was promoted to Senior Scientist, and Lee Bernstein was promoted to Faculty Senior Scientist.  After open searches, Weronika Wolszczak was promoted to Research Scientist, and Emil Rofors was promoted to Career Track Research Scientist.

Congratulations also go to Carol Chien who celebrates 10 years of service at LBNL!