Exploring Nuclear Structure

Hi, my name is Pete Christ and I am a rising senior working in the physics department this summer with Maria Mazza, a rising junior, and Dr. Stephenson. I am a recent addition to the group, considering that this is Maria’s second summer on the project. We are part of a group called the MONA Collaboration, which consists of undergraduate institutions from all over the country, as well as Michigan State University. This group was created to build and operate a Modular Neutron Array detector, (MONA), which is a large-area high efficiency neutron detector. MONA actually consists of 144 individual detector bars capable of detecting neutrons traveling at high velocities and is able to tell us position, trajectory and even the energy that each neutron has as it hits the detectors. LISA, (large multi institutional Scintillator Array) has also recently joined MONA, which is just another large neutron detector, which helps make readings even more accurate.

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So at this point you’re probably wondering why or what we use this for. Well, there are many different nuclear physics projects possible with access to this equipment (also a bunch of other detectors, huge sweeper magnets and a particle accelerator) and the data we collect from them. Many of these projects include looking at unstable isotopes and the limit at which isotopes become unbound. Unstable isotopes are elements with extra neutrons that decay very quickly (10-9 seconds) but unbound isotopes decay even faster (10-22 seconds). So one of the main goals of this group is to locate this line where the decay period drops off. This is called a drip line. Drip lines have been found up to oxygen on the periodic table (O26 is in fact unbound), but we still have a lot of isoptopes to discover before we can create a model explaining why and how these element’s isotopes exist. These types of isotopes are naturally produced only in supernovae explosion, but we are able to reproduce them in big laboratories using particle accelerators and detect them through neutron and particle detectors (that’s where MONA comes in).


The data that we are analyzing consists of a neutron-rich beam of Magnesium 32 (32 neutrons instead of 12) being directed into a target of Beryllium 9 gas. The collisions create very unstable isotopes that we call “prefragments” which decay emitting neutrons into the final fragments almost immediately. In order to see what these prefragments consisted of, we used a large sweeper magnet to direct the fragments containing protons into a set of detectors. Since these prefragments are moving at such incredible velocities, the neutrons that are emitted during the decay still have forward moving momentum and are eventually detected by MONA and/or LISA, which are straight ahead. By matching up the times of flight of neurtrons with the fragments redirected by the sweeper magnet, we can see what the prefragment must have consisted of before it decayed into different elements or isotopes. Since we are using Mg, the elements that we are looking into are primarily Sodium, Neon, and Fluorine because they have only a couple less protons than Magnesium.

Below are some examples of the data we analyze in order to gain a better understanding of prefragments and extremely unstable isotopes.

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