Researchers at Northwestern and Argonne National Laboratory have developed a new neutron detector that is both compact and effective.

Traditionally, many pre-existing neutron detectors consist of a semiconducting part with a neutron-absorbing layer on top. For this new neutron detector, the semiconducting material, lithium, is inside the device.

“We had the idea long ago that if we had a semiconductor that had lithium in the structure and the crystal structure, that material may be a good neutron detector,” Prof. Mercouri Kanatzidis, a Northwestern chemistry professor and lead researcher, said.

Lithium absorbs neutrons and then undergoes a nuclear reaction, generating a helium particle. The helium particle contains a lot of energy as it forms, exciting electrons into the semiconducting material. Those electrons could then be used to indicate the presence of the neutron.

Although the concept is simple, Kanatzidis said, it was difficult to put it into practice due to the unstable nature of lithium. Additionally, there are other conditions the semiconductor must fulfill. For example, Kanatzidis said electrons must also be able to travel through the material easily, which is difficult to achieve because most materials trap the electrons and prevent them from moving around.

“And there are other conditions where when there are no neutrons, the materials will absolutely conduct no electricity at all,” Kanatzidis said.

The crystals making up the semiconductor must also be the right size. If the crystal is too thick, the charge will dissipate into the crystal, but if it is too thin, fewer neutrons will be absorbed. Daniel Chica, a Northwestern graduate student and one of the paper’s co-first authors, was responsible for growing the crystals and was able to find the right balance so that the crystals met the required conditions.

“I developed this crystal growth technique called chemical vapor transport, where you essentially grow the crystals from the gas phase,” Chica said. “So they start at one side of the tube and then they kind of evaporate into the air of the reaction vessel and then condense on the other side.”

The device was first tested using alpha particles due to their accessibility and similarities in properties to neutrons, Assistant Prof. Yihui He, the paper’s other co-first author, said.

“So by using that alpha particle source, we determined the charge transport of the device to see if the device is suitable for the test or not,” He said. “It’s just a standard characterization.”

After testing the device with alpha particles, the group obtained permission to test the device with actual neutrons at Argonne National Laboratory. Despite the neutron beam being very weak, the crystals were still able to detect the neutrons.

This new neutron detector has applications to Homeland Security in that it can be easily transported to different places and help effectively find smuggled nuclear materials.

The group is currently also working on a device that detects gamma rays, another form of radiation that comes from nuclear materials.This will also involve growing crystals in the lab and synthesizing these materials.

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