(Inside Science) — Scientists have been attempting to measure the lifetime of a neutron outdoors an atomic nucleus for many years, and for the final 15 years, two sorts of laboratory experiments have offered completely different solutions. In a brand new examine, researchers for the second time have measured the neutron lifetime in a setting far outdoors the lab — space.
This new measurement — primarily based on knowledge captured close to the moon — is much less exact than the lab measurements, and it doesn’t make clear if both of the 2 lab outcomes is appropriate. But it exhibits it’s doable that future space-based measurements could be exact sufficient to assist reply the query of how lengthy neutrons survive. Robert Pattie, a physicist at East Tennessee State University in Johnson City who didn’t contribute to the brand new paper, known as the brand new outcomes “a neat experiment.”
Multiple measurements of a neutron’s lifetime
Neutrons are subatomic particles which can be usually secure after they’re inside an atom’s nucleus. When they’re outdoors, they decay in rather less than quarter-hour right into a proton, an electron and an antineutrino (an antimatter particle), however scientists aren’t precisely positive how a lot lower than quarter-hour it takes.
Measuring the neutron lifetime exactly is essential as a result of “it tells us things about fundamental properties of physics, across a really wide range of disciplines,” stated Shannon Hoogerheide, a physicist on the National Institute of Standards and Technology in Gaithersburg, Maryland, who didn’t contribute to the brand new paper. The relative abundance of hydrogen and helium within the universe shortly following the Big Bang, for instance, decided the sorts of stars and parts within the universe, and this hydrogen-to-helium ratio will depend on the neutron’s decay fee.
Scientists have measured the neutron lifetime to be 14 minutes and 39 seconds on common in a sequence of latest “bottle” experiments, which rely the variety of neutrons remaining over time, and 14 minutes and 48 seconds in the newest “beam” experiment, printed in 2013, which counts the variety of protons ensuing from decayed neutrons. The margins of error for these two sorts of experiments don’t overlap, so scientists have two solutions to the identical query.
Both solutions can’t be proper, so one or each experiments might need some design flaw that leads to the researchers mainly miscounting the neutrons and protons. There’s additionally a risk that some unknown physics could be contributing to the discrepancy, however Hoogerheide, who’s engaged on a brand new beam experiment, says that “a lot of people are probably leaning towards” it being a design downside.
Now, in a paper printed on-line Oct. 13 within the journal Physical Review C, a crew of researchers has measured the neutron lifetime utilizing knowledge from NASA’s Lunar Prospector, which orbited the moon for nearly 19 months between 1998 and 1999. The paper experiences a measurement of 14 minutes and 47 seconds, give or take about 16 seconds.
The final beam experiment had a margin of error of solely a little bit over two seconds, and a brand new bottle experiment was printed final week, with a margin of error of lower than half a second. The margin of error represents a spread of seemingly values for the neutron lifetime primarily based on the recognized limitations of the experiment. The 16-second margin of error of the space-based consequence pegs the lifetime as seemingly someplace between 14 minutes and 31 seconds and quarter-hour and three seconds — so it doesn’t rule out both lab consequence.
In future experiments, the margin of error will must be not less than as small as a few seconds with the intention to solely match one lab consequence. But this new paper reduces the margin of error considerably from the primary space-based neutron lifetime measurements, reported in a 2020 paper by the identical authors, elevating the chance that one other discount on the same scale could possibly be doable. “With some careful thought and careful design, maybe you could get [the margin of error] down to a low-enough level that it’s interesting from the point of view of the discrepancy,” stated Hoogerheide.
Measuring neutrons in space
Neutrons are touring by way of space close to the moon due to galactic cosmic rays — excessive power particles, similar to protons or helium ions, transferring near the pace of sunshine that come from sources outdoors the solar system, like supernovae explosions.
The moon has no ambiance, so these cosmic rays collide with its floor. Thomas Prettyman, a senior scientist on the Planetary Science Institute in Tucson, Arizona, stated that cosmic rays have a lot power that “when they collide with an atom, they produce an explosion where you get a spray of secondary particles that includes neutrons.” Prettyman didn’t contribute to the brand new examine.
Some of those neutrons journey up into space, and when the Lunar Prospector was orbiting the moon, some neutrons have been captured by its neutron spectrometer — a tube on the finish of a growth full of high-pressure gasoline that absorbs neutrons. This spectrometer, and the spacecraft’s different devices, weren’t attempting to measure the neutron lifetime — they have been in search of proof of water ice on the lunar poles. But within the absence of funding for a space mission particularly designed to reply the neutron lifetime query, researchers turned to those different knowledge units to see what they might discover.
“It’s always fun when you can take data that exists for some other reason and go learn something new with it,” stated Hoogerheide.
Jack Wilson, lead creator of the brand new paper and a planetary scientist on the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, first examined the concept of getting new details about neutron decay from outdated space missions in a 2020 paper neutron knowledge captured in 2007 and 2008 by NASA’s MESSENGER mission to Mercury.
“It hasn’t been something that large numbers of people have been expending a lot of effort on,” stated Wilson. “People just assumed it couldn’t be done.”
Wilson’s new paper modeled what number of neutrons the spectrometer ought to seize primarily based on quite a lot of components, together with the detector’s effectivity and the composition of the moon’s floor. Then, the researchers simulated what number of neutrons must be detected for various decay charges to see which decay fee greatest matched the detector’s knowledge.
Looking to Venus to enhance the outcomes
Wilson hopes that one in all a number of upcoming NASA missions to Venus would possibly convey alongside a neutron spectrometer to measure neutrons free of Venus’ ambiance, which is far more homogenous than the floor of the moon. The moon’s compositional complexity was the measurement’s essential supply of systematic error, Wilson stated. Systematic error could be brought on by issues like damaged gear or a lack of knowledge of the system being studied.
“Having a really good Venus data set would also give us, potentially, another order-of-magnitude improvement in the systematics,” he stated.
Hoogerheide thinks it is unlikely that the space-based measurements will ever be as exact because the bottle experiment. But Pattie, who works on bottle experiments, stated that “even if they can’t get [the margin of error] down to 0.1 seconds, if they can get down to a few seconds and say, We agree with the bottle or the beam… that’d be pretty interesting.”
Even if this have been to occur, the space-based measurement would solely be “more evidence that we’d have to keep looking until you figure out why the beam and bottle don’t agree,” stated Pattie.
Both Pattie and Hoogerheide assume the most definitely clarification for the discrepancy is a few kind of error in experimental design. But a couple of scientists have proposed that not less than some fraction of the discrepancy could be attributable to “new physics” — similar to some small share of the neutrons decaying into dark matter particles as an alternative of the protons detected by the beam experiment. But experiments haven’t discovered any proof in help of dark matter theories but.
“Everybody basically has always expected the anomaly to be systematic in nature,” stated Hoogerheide. “People always want to discover new physics, but don’t really ever expect to.”
But the space-based measurements might one day be used to say that one of many two lab measurements is extra seemingly than the opposite. Wilson stated the concept of attempting space-based measurements was interesting “just because this stalemate has existed between the two lab measurements… you want a third technique to break the stalemate.”
This story was initially printed with Inside Science. Read the unique right here.
