The neutron shell that surrounds the protons of the nucleus of a lead atom is 0.28 femtometers thick. A result that offers important information to understand the equation of state of nuclear matter, not only for atomic nuclei but also for neutron stars. We talk about it with one of the authors of the complex experiment, the physicist Guido Maria Urciuoli of the Infn in Rome
We can roughly imagine how an atom is made: around the electrons and in the center the nucleus, where protons and neutrons coexist. Yeah, but how do they coexist? How are they arranged? Scientists from the Prex-II experiment have now managed to measure the thickness of the neutron shell that forms the outermost layer of a core of lead atoms. The result – about 0.28 femtometers, a little higher than expected – was published Tuesday in Physical Review Letters , and also contributes to astrophysics, in particular to the understanding of some properties of neutron stars.
But how can we observe the arrangement of neutrons in a nucleus? Let’s take a step back, and take the periodic table in hand . Scrolling it element by element, and in particular looking at how the atomic number (the number of protons) and the “atomic weight” (to which the number of neutrons also contributes) progress, it is easy to see a trend: while the lighter nuclei, those with few protons typically have an equal number of protons and neutrons, as the nuclei become heavier the neutrons increase more rapidly than the protons. It is no coincidence: neutrons contribute to the stability of the nucleus. All stable nuclei with more than twenty protons have a higher number of neutrons than protons.
The most common isotope of lead (lead-208, the one studied with the Prex-II experiment), for example, has 82 protons and 126 neutrons. Arranged how? “The protons in a lead nucleus are in a sphere, and we found that the neutrons are in a larger sphere around them – we call it neutron skin ,” explains one of the experiment’s spokespersons, Kent Paschke , of the University. of Virginia (USA).
The first measures of the thickness of this “skin” date back to 2012 and come from the Prex experiment, the predecessor of Prex-II. These are extremely complex measures. Neutrons, unlike other subatomic particles, having no electric charge, cannot be detected through electromagnetic interactions. To determine its distribution, Prex therefore resorts to another of the four fundamental forces: the weak nuclear force. A beam of electrons spinning around themselves along the axis of displacement – a bit like a rugby ball in flight – is directed towards a thin sheet of lead cooled to cryogenic temperatures. Reached the lead nuclei, some electrons interact electromagnetically with the protons: a “symmetrical” interaction – underline the researchers – as it is independent of the direction of rotation.weak interaction , it is asymmetrical: it occurs more often when the spin is in one direction than the other.
And it is precisely by exploiting these differences and this asymmetry that the Prex experiment and – with greater precision – its successor Prex-II were able to measure the thickness of the shell. Thickness which is given by the difference between the total radius of the nucleus and the radius of the inner sphere of protons – the so-called charge radius , or charge radius . «The charge radius is about 5.5 femtometers, while the neutron distribution is a bit greater: about 5.8 femtometers. So the thickness of the neutron skin is 0.28 femtometers – that is 0.28 millionths of a nanometer. This is the most direct observation of the neutron shell to date, ”says Paschke, noting that this is a slightly higher value than they expected. “It means that we are in the presence of a rigid equation of state (stiff ), or a higher pressure than expected, such as to make it difficult to “crush” these neutrons inside the nucleus. In other words, we are discovering that the density inside the nucleus is a little lower than expected ».
And why the lead? The technique used, in fact, is not limited to lead cores. Recently it has also been used for calcium , for example, to be precise on the 48Ca isotope. But lead lends itself very well to this type of measure. “Lead is a stable element, so it is” easily “used in a laboratory as a target for electrons that affect it”, explains to Media Inaf another spokesperson for the Prex-II experiment, the physicist Guido Maria Urciuoli, researcher of the Rome section of the Infn. “Lead has the property of having a large difference between the number of neutrons and the number of protons and, therefore, was an excellent candidate to verify – for the first time, independently of the models – the existence of a” skin of neutrons ”, or of a difference (supposedly positive, as indeed it has been verified) between the neutron beam and the proton beam in the nucleus. The quantitative measure of this difference is linked to the equation of state of matter rich in neutrons, an equation on which the characteristics not only of the nuclei of the elements we have on Earth depend, but also those of those gigantic nuclei that are the neutron stars. Obviously, the lead nucleus is the nucleus which by characteristics – excess of neutrons compared to protons,
Therefore a result of considerable interest also for astrophysicists, precisely because of the consequences that this measurement can have for the understanding of the equation of state of neutron stars. “When two neutron stars start spiraling around each other, they emit gravitational waves like those detected by Ligo”, remembers another scientist from the Prex-II team, Krishna Kumar.of the University of Massachusetts Amherst. “In the last split second of their approach, the gravitational pull of one neutron star deforms the other neutron star into a ‘tear’ – actually, it makes it oblong, a bit like an American football. If the neutron skin is thick, this shape will be different from what the neutron star would take if the neutron skin were thinner. ‘ Perhaps the most interesting aspect, continues Kumar, is that «Ligo is able to measure the ‘shape of the ball’. The Ligo experiment and the Prex experiment have done very different things, but they are connected by this fundamental equation: the equation of state of nuclear matter ».
Featured image: Representation of a 208 lead nucleus showing the mixed proton-neutron nucleus and the neutron “skin” (top). Neutron skin thickness measurement offers clues to how neutron stars are structured (bottom). Credits: Aps / Alan Stonebraker
To know more:
- Read on Physical Review Letters the article “ Accurate Determination of the Neutron Skin Thickness of 208Pb through Parity-Violation in Electron Scattering“, By D. Adhikari, H. Albataineh, D. Androic, K. Aniol, DS Armstrong, T. Averett, S. Barcus, V. Bellini, RS Beminiwattha, JF Benesch, H. Bhatt, D. Bhatta Pathak, D Bhetuwal, B. Blaikie, Q. Campagna, A. Camsonne, GD Cates, Y. Chen, C. Clarke, JC Cornejo, S. Covrig Dusa, P. Datta, A. Deshpande (11 and 14) D. Dutta, C. Feldman, E. Fuchey, C. Gal (11 and 15 and 14), D. Gaskell, T. Gautam, C. Ayerbe Gayoso, M. Gericke, C. Ghosh (17 and 11), I. Halilovic , J.-O. Hansen, F. Hauenstein, W. Henry, CJ Horowitz, C. Jantzi, S. Jian, S. Johnston, DC Jones, B. Karki, S. Katugampola, C. Keppel, PM King, DE King, M. Knauss, KS Kumar, T. Kutz, N. Lashley-Colthirst, G. Leverick, H. Liu, N. Liyange, S. Malace, R. Mammei, J. Mammei, M. McCaughan, D. McNulty, D. Meekins, C . Metts, R. Michaels, MM Mondal (11 and 14) J. Napolitano, A. Narayan, D.
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