There are many different characteristics of rare isotopes that scientists would like to measure and – consequently – just as many techniques needed to do the job. As the field progresses by unlocking more and more of the rarest of isotopes’ secrets, often it becomes necessary to create new techniques and upgrade the equipment to push the boundaries. After all, the easiest isotopes to create and study are typically the first ones to be explored. At NSCL, the faculty, staff and students are in a constant battle to make new experiments possible before others beat them to the punch.
Recently, the lab unveiled two new instruments in a single experiment designed to measure the probability of creating copper-56 nuclei from nickel-56 nuclei via charge-exchange reactions. In a charge-exchange reaction, a neutron in nickel-56 is replaced by a proton to create copper-56. By measuring the velocity of the reaction products, the experimenters can learn about the details of the reaction like how much energy was transferred. However, hitting any nucleus with a proton and seeing the results isn’t as easy as it sounds.
And I bet you didn’t think it sounded very easy at all.
A nucleus is only a very tiny fraction of an atom. In fact, if a hydrogen atom – which is basically just a single proton with a single electron – were the size of a football field, the nucleus would be only about as thick as a sheet of double- ply toilet paper. Because most of an atom is just empty space, hitting two tiny nuclei together is very difficult. To get around this problem, scientists play the percentages. By throwing billions of atoms at a very dense target, they raise the odds of creating a collision.
The first new instrument used in the experiment was a target made of liquid hydrogen. Historically, targets are made out of the substance being studied, which are bombarded by small probe nuclei such as hydrogen. But because nickel-56 only lasts for about six days, creating a target stable enough to last the length of the experiment wasn’t possible. Instead, the experiment took a beam of nickel-56 and crashed it into a target of hydrogen.
Using a liquid form of the target, rather than a gas, provided the experimenters with a high density of hydrogen atoms as well as a pure target. If they had used a plastic target, which is made out of hydrogen and carbon, it would not have been as dense. Plus, isotopes would hit carbon instead of hydrogen occasionally, creating background clutter that would have to be weeded out afterward.
Liquid hydrogen is a finicky substance to work with. In order to stop it from turning into a gas, it must be kept below 20 degrees Kelvin; that’s -423 degrees Fahrenheit to the non-scientific world. There are commercial products available for this purpose, so the low temperatures weren’t the issue.
“The challenge was in keeping it stable,” said Remco Zegers, an associate professor at NSCL and leader of the experimental group running the experiment. “You have to make sure the foils sealing in the hydrogen don’t break and that you can maintain it for a long time because the experiment takes up to a week to run.”
By reducing the background noise and having a dense target, the liquid hydrogen target improves performance over plastic targets by a factor of 10. It was based on a design from a Japanese nuclear physics group and was constructed over the summer by researchers from Ursinus College and the NSCL, supported by a research grant from the National Science Foundation (PI: Prof. Lew Riley).
The device worked beautifully. The beam of nickel-56 came in at nearly half of the speed of light, and collided with the hydrogen atoms - protons -in the liquid hydrogen target. The heavy reaction products -copper-56 -were detected in an already existing detector system at the NSCL. However, the neutrons generated in the charge-exchange reaction had such a low amount of energy that no instruments at the lab could detect them.
Neutrons are difficult to detect and measure in the first place. Because they are electrically neutral, they are much harder to stop and to be detected than protons or other charged particles. To solve this problem, scientist create materials in which incoming neutron can collide with a high probability with charged particles, which are easily stopped and create detectable signal. Scientists use scintillating materials for this purpose; when the particles get stopped they generate a pulse of light.
NSCL already has a couple of neutron detectors, including the Modular Neutron Array (MoNA), which was designed and built by undergraduate students from multiple institutions. However, MoNA is designed to detect highly energetic neutrons, which cause brighter flashes of light. For this experiment, the scientists needed to detect neutrons with about 100 times less energy.
To do this, a team at the NSCL designed and constructed the Low Energy Neutron Detector Array, or LENDA. The new device consists of 24 bars made of scintillating plastic. The bars are smaller than MoNA’s, so that even the smallest flashes aren’t lost. Additionally, the light sensors at the ends of the tubes are extremely sensitive.
“You basically do everything you can to catch the light of even the smallest interaction between the neutron and the scinillator,” said Zegers.
The 24 bars are split into two groups of 12 standing vertically side-by-side. The neutron that is produced in the charge-exchange reaction can be ejected at various angles, so the detector array must cover quite a large area.
All in all, the debut of these two new pieces of equipment went off without a hitch. Both performed as they were supposed to. Now the experimenters just have to make sense of week’s worth of continuous data they supplied.
Once again, easier said than done.