mars.nasa.gov's CheMin for Scientists (found here) says (about half-way down):

Detection of X­-ray Photons by the CCD

CheMin will use a 600 × 600 E2V CCD-224 frame transfer imager operated with a 600 × 582 data collection area. The pixels in the array are 40 × 40 µm2, and the active region of deep depleted silicon is 50 µm thick. The front surface passivation layer is thinned over a substantial fraction of the active pixel area. This imager is a modern version of the E2V CCD-22 that was specially built for an X-ray astronomy application. The large size of the individual pixels causes a greater percentage of X-ray photons to dissipate their charge inside a single pixel rather than splitting the charge between pixels. The enhanced deep depletion zone results in improved charge collection efficiency for high energy X-rays. The thin passivation layer makes the CCD sensitive to relatively low-energy X-rays.

In order to keep the CCD from being exposed to photons in the visible energy range (from X-ray induced optical fluorescence) during analysis, a 150 nm Al film supported on a ~2,000-Angstrom polyimide film is placed in front of the detector. The detector itself is cooled to a target temperature of minus 60°C, but the actual CCD temperature will depend on the rover body upper-surface temperature. By cooling the CCD, dark current is eliminated, and the effects of damage to the silicon lattice by neutrons from the Radioisotope Thermoelectric Generator (RTG) and the DAN science instrument will be reduced. Should the temperature not reach minus 60°C for the analysis, the dark current will increase and the neutron damage to the CCD will begin to adversely affect Charge Transfer Efficiency (CTE), resulting in higher background counts and increased full width half maximum (FWHM) in X-ray peaks.

The Russian "DAN science instrument" or Dynamic Albedo of Neutrons generates neutrons and illuminates the Martian surface with them so concern for crystal damage to the relatively thick CCD active volume is not surprising:

The Dynamic Albedo of Neutrons (DAN) instrument is an experiment mounted on the Mars Science Laboratory's Curiosity rover. It is a pulsed sealed-tube neutron source and detector used to measure hydrogen or ice and water at or near the Martian surface. The instrument consists of the detector element (DE) and a 14.1 MeV pulsing neutron generator (PNG). The die-away time of neutrons is measured by the DE after each neutron pulse from the PNG. DAN was provided by the Russian Federal Space Agency, funded by Russia and is under the leadership of Principal Investigator Igor Mitrofanov

But I'd never heard of neutrons from an RTG before.

Curiosity has an MMRTG; a Multi-mission radioisotope thermoelectric generator based on 238Pu. According to Plutonium-238; Nuclear powered pacemakers we used to put 238Pu inside people to power life-giving pacemakers, so it can't be a strong neutron source, and the whole idea is that the very short range alpha particles stop within the material itself an thermalize, making it a fairly radiation-free source that you can keep around astronauts and sensitive electronics.

Neutrons can come from both nuclear fission and beta-delayed neutron emission: (1, 2, 3, 4).

Question: Does curiosity's RTG generate neutrons as this NASA CheMin X-ray detection system webpage suggests? If so, since 238Pu decays by alpha particle emission, where are the neutrons coming from?

  • 2
    $\begingroup$ Pu-238 is only "best-effort" clean from other Pu isotopes. Pu-239 is probably present in high amounts. $\endgroup$
    – fraxinus
    Jun 4, 2021 at 13:31

1 Answer 1


There are two major sources of neutrons produced in $^{238}\rm Pu$-based RTGs:

  • $^{238}\rm Pu$ mainly decays by emitting an α particle, but there is a $10^{-8}$ chance of spontaneous fission creating all kinds of messy products, including neutrons.
  • The fuel of RTGs is not pure Plutonium, instead $\rm PuO_2$ is used because it is more stable (in the chemical sense). The α particles can interact with the Oxygen and kick out neutrons from its nucleus.

The interaction of α particles with light nuclei contributes about 90% of the total neutron emission, which is about $10^{-7}$ of the primary alpha emission. Nevertheless, this low relative rate sums up to a substantial amount of radiation: Perseverance uses 4.8kg fuel, with an activity of $5\cdot10^{11}/\rm g/s$, resulting in $10^9$ neutrons per second in total.

@fraxinus mentioned impurities. Indeed, $^{239}\rm Pu$ is contained to a substantial amount (10-15% as it seems). Luckily its emitting α particles as well, but at a low rate due to its longer lifetime.

main source:

  • 3
    $\begingroup$ (I'm sure you know this, but for the benefit of other readers), $^{238}\rm Pu$ is made by neutron irradiation of $^{237}\rm Np$, so some $^{239}\rm Pu$ is inevitable. And trying to separate them is much harder than separating $^{235}\rm U$ & $^{238}\rm U$. $\endgroup$
    – PM 2Ring
    Jun 4, 2021 at 16:56
  • 1
    $\begingroup$ Don't forget the activity of the daughter nuclei as well. $\endgroup$
    – Jon Custer
    Jun 4, 2021 at 17:12
  • 1
    $\begingroup$ @JonCuster - That's not too bad. The first few products do α decay as well, and have long half lives. You need to get down to $^{214}\rm Pb$ for beta decay to happen. $\endgroup$
    – asdfex
    Jun 4, 2021 at 17:33
  • 2
    $\begingroup$ Both $^{234}$U (from $^{238}$Pu) and $^{235}$U (from impurity $^{239}$Pu or neutron absorption by $^{234}$U) have spontaneous fission decay paths, providing neutrons which could be of concern here. $\endgroup$
    – Jon Custer
    Jun 4, 2021 at 17:36
  • 2
    $\begingroup$ @asdfex - indeed. Just I've seen too many times where people forget the daughter product activities. $\endgroup$
    – Jon Custer
    Jun 4, 2021 at 17:44

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