New experiments view details of nuclear two-proton emission

Were it not for the strong force, a nucleus would be blown apart by electrostatic repulsion. Viewed as an interaction between fundamental quarks, the strong force is well understood. The protons and neutrons that make up a nucleus, however, are composite particles. And to this day, physicists are struggling to determine the basic nuclear interactions responsible for holding the nucleus together (see the article by David Dean, Physics Today, November 2007, page 48 ).

To that end, nuclear physicists often turn to barely bound nuclei, whose unusual properties and exotic decay modes can test theoretical models and thus provide valuable clues about nuclear forces. One intriguing example is proton-rich iron-45, whose lifetime is a few milliseconds. On occasion, the isotope emits two of its protons simultaneously. The signature of that decay, though not the two protons themselves, was observed in 2002, in experiments performed at the Laboratory for Heavy-Ion Research in Darmstadt, Germany, and at GANIL, the French national heavy-ion accelerator in Caen (see Physics Today, September 2002, page 17 ).

Now follow-up experiments have directly observed the two protons emitted by ⁴⁵Fe. This past September, Bertram Blank and colleagues working at GANIL reported the first direct observation of the two decay products. ¹ And in November, Krzysztof Miernik, Wojciech Dominik, group leader Marek Pfützner, and an international team published results of an experiment that tracked the emitted protons and measured their angular correlation. ² Both groups adapted a venerable technology familiar to particle experimentalists—the time projection chamber.

The figure on page 27 shows a typical two-proton event, obtained at Michigan State University’s (MSU’s) National Superconducting Cyclotron Laboratory. Pfützner and colleagues’ measurements rule out the simplest models purporting to describe the two-proton decay. But they are consistent with a more complicated phenomeno-logical model proposed by Mikhail Zhukov and team member Leonid Grigorenko. ³

Two for the show

It seems plausible that a just-bound nucleus with an excess of protons might lower its energy and approach stability by shedding a proton, perhaps a few. But what is the advantage in emitting two protons at once? Vitaly Goldansky, who predicted two-proton decay in 1960, reasoned that in a nucleus with an even number of protons, the strong force leads to a pairwise binding of all the protons. Emitting a single proton entails breaking a nuclear bond and, possibly, a net energy gain. Ejecting two protons simultaneously and not breaking a bond in the nucleus could then be the energy-lowering option.

Once the two protons are out of the nucleus, though, they will separate. The details of that breakup preserve a memory of the state of the two protons in the nucleus. In one extreme theoretical case, the pairing of the two protons requires that what emerges is a single entity—a diproton. In that scenario, however, the two protons resulting from the breakup of the diproton would have a sharply peaked angular distribution that is inconsistent with the MSU and GANIL measurements.

Three to get ready

Rather than leaving the nucleus as a single body, the two protons emerge as distinct units. Along with the nucleus itself, the protons participate in a three-body decay. “Grigorenko’s is the only three-body model presently on the market that predicts the protons’ angular distributions,” observes Pfützner. The input to the model is a presumed wavefunction for the state of the two protons in the ⁴⁵Fe nucleus. In addition to predicting angular correlations, the model relates the total energy carried away by the two protons to the lifetime of the decaying ⁴⁵Fe. In all respects, it is consistent with the results obtained by Pfützner and company.

To determine the trajectories of the decay protons, both the Pfützner and Blank groups developed their own time projection chambers. TPCs are filled with an ionizable gas and a strong vertical electric field. When a particle such as an ⁴⁵Fe ion passes through the chamber, it ionizes the gas; the freed electrons then drift downward. By measuring the horizontal coordinates of the electrons (the “projection” in TPC) and timing how long it takes them to drift through the TPC’s electric field, one can reconstruct the three-dimensional path of the ionizing particle. The TPC used at MSU included a new wrinkle developed by Dominik: The ionized gas ultimately created a shower of visible photons that enabled Pfützner and company to photograph the trajectories of the ⁴⁵Fe and decay-product protons.

TPCs have long been successfully employed in particle-physics experiments; still, the MSU and GANIL teams faced considerable hurdles to adapt the device for nuclear physics. For one thing, in particle-physics experiments, all measured objects typically have similar ionizing powers. But the ⁴⁵Fe in the nuclear experiments is a much stronger ionizer than a proton and could overwhelm electronics that are optimized to track proton paths. The TPCs developed for the two-proton observations had to be nimble enough to handle two very different types of ionizer.

With TPCs in hand, the two experimental groups will explore other nuclei with exotic decays. One nucleus that both collaborations intend to study is the two-proton emitter zinc-54, which differs from ⁴⁵Fe in a potentially telling way. Zinc has 30 protons, 28 of which fill nuclear shells analogous to atomic orbitals. When the extra two protons in ⁵⁴ Zn are emitted, they leave behind a magic nucleus with fully filled proton shells. By contrast, ⁴⁵Fe decay results in a nucleus with a partially filled shell.

Already the Pfützner team’s ⁴⁵Fe runs have provided a glimpse of a decay never previously seen. ⁴ In 4 of their 125 events, the group observed the rapid emission of three protons from an excited nucleus.