Nuclear Masses Measured to within a Hair’s Precision

No one likes to say exactly how much they weigh. Rare atomic nuclei are similarly coy, obviously not because of their own volition, but rather because they are exceedingly difficult to produce and, while they exist, very short-lived and difficult to corral and accurately measure.

Now, MSU researchers have made precise mass measurements of four such nuclei, 68-selenium, 70-selenium, 71-bromine and an excited state of 70-bromine (yes, a nucleus weighs measurably more when it is excited because of Einstein’s famous E=mc2 declaration). The results may make it easier to understand X-ray bursts, the most common stellar explosions in the galaxy.

X-ray bursts are spectacular runaway thermonuclear reactions on neutron stars that release vast amounts of energy in a short period of time. In just 10 seconds, an X-ray burst might release as much energy as our sun does in one month. Such explosions occur in binary systems where a neutron star and a second donor star orbit each other. The donor star rains hydrogen and helium onto the surface of the neutron star. When enough of this material accumulates, nuclear fusion reactions begin, dramatically increasing temperature to nearly 2 billion degrees Fahrenheit, which is about 10,000 times hotter than the surface of the sun. This temperature spike gives rise to the explosion and eventually to what’s known as the rapid proton capture nucleo-synthesis-process, or rp-process.

The rp-process occurs when a seed nucleus in a super-hot stellar environment begins capturing protons in quick succession, piling them up until the nucleus cannot hold any more. The nucleus then spits out some energy, turning a proton into a neutron, which allows the piling on to start anew.

The rp-process is roughly analogous to stacking blocks one after the other. Eventually the stack gets sufficiently tall and unsteady that the blocks fall into a more compact and stable jumble. If the stacking continues on top of this pile, eventually a new jumbled shape will be created when the blocks fall down a second time. In time, this repeated stacking and tumbling will create a slew of new increasingly larger piles, just as the successive capture and decay during the rp-process is thought to create many heavy elements, possibly up to tellurium, stable versions of which have 52 protons and anywhere from 70 to 74 neutrons.

The MSU team, including nuclear science doctoral student Josh Savory, were interested in four atomic nuclei because they represent a pause button of sorts during the rp-process. Normally the capture-decay sequence that creates new elements happens in a blink of an eye, in a matter of seconds or less. However it takes time, perhaps 30 seconds or more, for selenium-68 and a few similar nuclei to decay. It’s possible these waiting points can be bypassed if two protons are captured instead of one. Precise mass measurements help to refine theoretical models that explain whether or not these waiting points are bypassed and in general, just how fast nuclear reactions proceed during X-ray bursts. This information, in turn, helps researchers predict and explain just how much of each of the various elements are produced during the rp-process.

The experiment, conducted by Savory and several colleagues, used NSCL’s Low Energy Beam and Ion Trap facility, LEBIT, to make the mass measurements of the four nuclei. LEBIT uses a technique known as Penning trap mass spectrometry to perform these measurements. (A physics 101 aside: Weight and mass are often confused. Weight of matter is entirely dependent upon the strength of gravity while the mass of matter is constant. Someone who weighed 180 pounds on Earth would weigh just 30 pounds on the moon, which exerts a much more modest gravitational pull. That same person’s mass would be the same on Earth, the moon or, with few exceptions, anywhere in the universe. The equation is w (weight) = g (gravity) X m (mass)).

LEBIT takes isotope beams traveling at roughly half the speed of light and carefully slows and stops the isotopes for highly accurate mass measurement. MSU is home to the only physics lab in the world capable of performing such measurements on isotopes produced by fast beam fragmentation, a technique that allows for the production of extremely rare nuclei not normally found on Earth.

The MSU team measured the masses to a level of precision as high as 1 part per 100 million (for 68-selenium) and with an improved precision as large as 100 times (for 71-bromine) in comparison to previous such measurements.

“As an analogue, think of a scale precise enough to see how your weight changes when you pluck just one hair out of your head,” said Savory, lead author of a paper describing the results which appears in Physical Review Letters.