Science AMA Series: I’m Sara Pozzi, the director of the Consortium for Verification Technology (CVT) and a Professor of Nuclear Engineering and Radiological Sciences at the University of Michigan. My group, and the CVT, develop technologies for nuclear nonproliferation. AMA!


My research group, the Detection for Nuclear Nonproliferation Group, works on technologies for detecting special nuclear materials, such as uranium-235 and plutonium-239, that can be used to make nuclear weapons. We focus on scintillators that can detect both neutrons and gamma rays, and which operate at room temperature.

Having developed algorithms to reliably tell the more informative neutrons from the much more common gamma rays, we are now exploring how to turn scintillator arrays into radiation imagers. Also, with the benefit of a new X-ray generator, we will be investigating the use of our detectors in “active interrogation” - using radiation to make special nuclear materials more radioactive so that they can be measured more quickly and accurately.

Our detection strategies, and those in development by our colleagues in the CVT, are designed to ensure that nuclear treaties are being followed. This means watching for signs that nuclear energy countries are secretly trying to become nuclear weapon states, detecting undeclared nuclear tests and confirming - without revealing classified information - that countries are disarming old warheads.

I will be back at 10 am ET to answer your questions, AMA!

What do you make, within the confines of what you're able to show legally, of the recent incident in Russia involving ruthenium-106 detections?


The recent incident with ruthenium-106 in Russia was quite interesting. ruthenium is one of many fission products. The event shows the technical difficulty in identifying if a detection is due to civilian processes (e.g. medical isotope production, regular nuclear power facilities, etc.), or other activities. Additional signatures from other isotopes are required to uniquely determine the source of the emission. We are still analyzing the data from the event.

1) What are the main benefits of room temperature operation?

2) Are you contracted by governments on a case by case basis or do you have a monitoring network set up that is constantly running?

3) How many federal agents stand over your shoulder when you're discussing this with the public :)


Room temperature detectors are easy to use in the field (when compared to detectors that need to be cooled). The International Monitoring System (IMS) was put into place by the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). Our group works to improve the detection capabilities of radioxenon monitoring which are vital to identifying nuclear explosions. No federal agents here, we are a university so we work in non-classified areas of research. However, some data that we receive from our national lab partners must go through approvals before being published.

What is a scintillator exactly and how does it work?


Scintillators work by converting the energy transferred to the detector into visible light (hence the name "scintillator"). This light is converted into a voltage pulse that can be measured by a device such as an oscilloscope. The light production varies by the composition of the scintillating material, of which there are many. Some scintillators are very interesting to us because they have the ability to detect and discriminate between the different types of particles, such as neutrons and gamma rays.

Hi Sara!

I have very little experience in the homeland security aspect of nuclear physics, so maybe my questions are going to be very stupid.

So... How are these detectors going to be used? Neutrons don't travel very far, so I imagine that you need to put them directly on site for gammas and most likely within a couple of meters for neutrons. Basically, we can check just places where we suspect something's going on and were the enemy state gives us access to.

I guess things are different than how I make them!! Can you give us an overview of how monitoring actually works?


In research for preventing the spread of nuclear weapons, we develop ways to monitor accessible facilities (such as nuclear reactors, fuel storage facilities, reprocessing facilities, etc.) and inaccesible facilities (such as those in North Korea). Neutron monitoring is generally applied in accessible facilities. For example, we can measure neutrons from plutonium to determine the mass of plutonium, to make sure that none of it was stolen over time. For inaccessible facilities, other radiation detection techniques are more effective, for example detecting radioactive products of nuclear explosions, such as xenon or krypton that are not easy to be shielded or blocked. These gases can travel thousands of miles, and be detected at very low concentrations. For example, we detected radioactive xenon emissions from the 2013 North Korean nuclear test.

Hi Sara, I have a final question for you!

You say that you're interested in detecting undeclared nuclear tests. I was wondering, how do they work?

I mean... Are they fizzles or is someone actually able to hide the seismic signature?


There is a system put in place by the Comprehensive Nuclear test Ban Treaty Organization (CTBTO) called the International Monitoring System (IMS) which uses the technologies of seismic, infrasound, hydroacoustic, and radionuclide monitoring to identify undeclared nuclear testing. More details of the IMS are found at In the event of a seismic event which is difficult to conceal with the vast amount of detectors in place, the monitoring stations also look for additional signatures to verify nuclear testing. A fizzle is when a nuclear test does not quite work as expected (for example the design of the weapon is not optimized). The fizzle sometimes does not produce enough signal to be detected. However, we know that the first North Korean test in 2006 was a fizzle and we were able to detect it.

How close do your devices have to be to the point of origin of a radioactive material to pick up traces of it? Thanks!


Great question! The answer is really dependent on the application. In some cases, we may be able to inspect radioactive materials up close, and tailor our detectors for that inspection (i.e. for accessible facilities). For undeclared activities, such as covert nuclear weapons tests, radioactive gaseous products may be detected thousands of miles from the source.

Dear Sara,

I have a second question regarding your work. You say you've been working to turn your scintillators into "imagers". How does this exactly work? What kind of objects are you imaging?

BTW, wouldn't you happen to be looking into hiring another person from Milan with a very strong background in tomographic reconstruction who's sick and tired of Italian research salaries?


To make an imager, we need multiple detectors; their response can be correlated in time and energy. This allows us to detect the presence of plutonium, for example. Here are some technical details: The organic scintillators that we have been developing are able to detect both neutrons and gamma rays. The primary detection mechanism in these scintillators involves a scatter-based interaction, which can offer both timing and energy information. Having two arrays of these scintillators allows us to project many probable source locations (i.e. backprojection cones), and through advanced imaging algorithms, we have demonstrated the ability to image the location of radioactive sources that emit both neutrons and gamma rays, as well as the ability to image special nuclear material. Furthermore, we have also demonstrated that we can isolate different source locations based on their energy. If you would like some more technical background on our radiation imaging work, here is a science journal article that summarizes this research topic: M. C. Hamel, J. K. Polack, A. Poitrasson-Rivière, S. D. Clarke, S. A. Pozzi, “Localization and spectral isolation of special nuclear material using stochastic image reconstruction,” Nuclear Instruments and Methods in Physics Research Section A, vol. 841, pgs. 24-33, 2017.

Dear Sara,

I have a second question regarding your work. You say you've been working to turn your scintillators into "imagers". How does this exactly work? What kind of objects are you imaging?

BTW, wouldn't you happen to be looking into hiring another person from Milan with a very strong background in tomographic reconstruction who's sick and tired of Italian research salaries?


Here is a youtube video on our imager:

Has your research funding been impacted by the Trump administration?


We are fortunate that our funding has not been affected, so far.

How are these technologies used in larger international policy agreements such as the JCPOA?


The Joint Comprehensive Plan of Action’s (JCPOA) main objective is to ensure that Iran’s nuclear program will pursue only peaceful uses. The JCPOA, for the first time, allowed International Atomic Energy Agency (IAEA) inspections in Iran’s facilities. This is great result for the international community. Inspections are the first way of verifying that no weapons grade plutonium or enriched uranium are produced in quantities that could allow the construction of a nuclear weapon.

Do your technologies ever get confused by particles of a cosmic origin?


That’s a great question! And in short yes! Specifically, we are constantly surrounded by background cosmic radiation, which can be detected by our technologies (this is why physics experiments, which use very similar detector technologies, are typically conducted deep underground to mitigate cosmic effects). However, we have investigated many different ways to discriminate between the cosmic-induced signatures and those signatures that arise from radioactive materials of interest. Furthermore, the signatures we are looking for from special nuclear material are very characteristic and different than those from cosmic origins, which further helps us with the discrimination.

Who funds this kind of research? What kind of granting agencies deal with this? DOE?


Our research is typically funded by the National Nuclear Security Administration (NNSA), which is a part of the Department of Energy, the Department of Homeland Security, and the Defense Threat Reduction Agency, which is part of the Department of Defense.

Does the new X-ray generator cause false positives?


The new X-ray generator will be used to clear up false positives in the assay of cargo containers that are "suspicious". These cargo containers can be sent to secondary inspection where the X-ray generator could be deployed.

I'm assuming what's going on with North Korea right now is beyond the scope of your work... but do you have any insights you might share?


The research our group conducts is directly related to the events in North Korea: we are developing more sensitive detectors to help monitor nuclear explosions. Through our Consortium for Verification Technology (, we are able to study the seismic signatures of the six North Korean nuclear tests over the years. These signatures clearly demonstrate that the magnitude of the tests are growing. Specifically, the last test was about a factor of 30 greater than the previous test. For context, the bombs dropped on Hiroshima and Nagasaki were order of 15-20 kT, whereas the 2017 North Korean test had a yield of approximately 200 kT.

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