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The answers to the Space SE question Soyuz landing ground detection really surprised me. A gamma ray altimeter is used by the returning Soyuz capsule to detect the presence of solid ground immediately below the capsule at the last split-second (not foliage or other weak structures) to trigger the retro-rockets milliseconds before touch down to reduce the impact.

What kind of sensor is used to reliably detect gamma rays scattered from the Earth? It would have to be fairly sensitive because only a small fraction of the original gamma ray intensity would backscatter and find its way back to a small detector with enough energy to make it through the heat shield attenuation. It would also have to very reliably survive all of the vibrations and thermal excursions along the way.

Wikipedia (Russian): Гамма-лучевой высотомер

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    $\begingroup$ It feels this is very critical piece of technology as googling gamma ray altimeter didnt help. So this thing's principle in not readily available in literature ! $\endgroup$
    – zephyr0110
    Apr 26, 2018 at 13:14

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It's called the "Kaktus-2".

The sensor is referred to as a "NaI(TI) crystal detector" which appears to be a scintillation counter.

The device has a 2 of 3 voting scheme to avoid false positives.

"...the source emits 13.7 Sv/hr point blank at 1 cm, and 1.3 mSv/hr at one meter. That's quite hot :)"

Here's a picture of the device.

enter image description here

And here's a block diagram.

enter image description here

The original Kaktus-1 used a different isotope and had a 5 channel voting scheme.

Source 1 (google translated Russian page)

Source 2

Source 3

Source 4

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  • $\begingroup$ Why is info not available in literature? $\endgroup$
    – zephyr0110
    Apr 26, 2018 at 13:13
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    $\begingroup$ I've made a small edit... $\endgroup$
    – uhoh
    Apr 26, 2018 at 13:42
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    $\begingroup$ ...so it just continuously irradiates the capsule from installation onward? wow $\endgroup$
    – Erin Anne
    Apr 28, 2018 at 3:13
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    $\begingroup$ @ErinAnne that's a good point. Since the isotope source itself can be quite small, perhaps a few mm^3, several centimeters of lead shielding can be wrapped directly around it on the crew-facing side (if necessary) without adding a large amount of mass. See some of the images in web.vu.lt/ff/a.poskus/files/2013/06/NP_No10.pdf for example. After shielding, it may not be large compared to the total radiation does the astronauts receive from the duration of their time in space anyway. $\endgroup$
    – uhoh
    Apr 29, 2018 at 5:32
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    $\begingroup$ Not to doubt the Soyuz engineers! Their track record speaks for themselves. But it seems.....a very Russian thing to do. $\endgroup$
    – Erin Anne
    Apr 29, 2018 at 5:56

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