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Why hydrogen shields best

Radiation and the atom · level 7, lesson 4 of 6

A question to start

For gamma rays, a gram of lead and a gram of water are about equal. Against cosmic rays, NASA says water and plastic are far better than metal, gram for gram. Why?

The idea

NASA's engineers put it simply: the best way to stop a fast particle is to run it into something of a similar size. A small particle hitting a big nucleus just bounces off, keeping its energy. The protons and neutrons in space radiation are best stopped by hydrogen, whose nucleus is a single proton.

Hydrogen helps in three ways:

  • Head-on hits. A proton or neutron can hand much of its energy to a hydrogen nucleus in one collision.
  • More electrons per gram. A hydrogen atom has one electron for each proton, with no neutrons adding weight. So a gram of hydrogen has about twice as many electrons as a gram of most other elements, to slow charged particles.
  • Fewer secondaries. A hydrogen nucleus cannot be smashed into fragments. Big nuclei, like those in metals, break up into showers of secondary radiation when cosmic rays hit them.

So the best shields are rich in hydrogen: water and plastics like polyethylene. A crew must carry water and make trash anyway, so both can be put in the walls. NASA is also testing hydrogenated boron nitride nanotubes, a strong material with hydrogen packed between the tubes and boron to soak up neutrons.

Material Hydrogen, by mass Electrons per gram, compared with lead
Polyethylene 14% 1.44
Water 11% 1.40
Aluminium 0% 1.22
Lead 0% 1
same size: energy handed onbig nucleus: bounces off

Worked examples

1. Hydrogen in water. H₂O has mass \(2 + 16 = 18\). Hydrogen is \(2 / 18 \approx 11\%\) of it.

2. Electrons per gram. Water has 10 electrons for a mass of 18: \(10/18 = 0.56\). Lead has 82 for 207: \(0.40\). \(0.56 / 0.40 = 1.40\), so water has 40% more electrons per gram.

3. A water wall. A layer 10 cm thick has 10 g/cm², 100 kg over each square metre of wall. Around a small storm shelter, a crew's drinking water could do the job.

Try it

  1. Why does a fast proton lose more energy hitting hydrogen than hitting lead?
  2. Find the share of hydrogen, by mass, in polyethylene, CH₂ (carbon 12).
  3. Why does a metal shield make more secondary radiation than a plastic one?
  4. Give two things a crew carries anyway that could be used as shielding.
Answers
  1. The hydrogen nucleus has about the same mass, so it takes much of the energy; a lead nucleus is so heavy the proton just bounces off.
  2. \(2 / 14 \approx 14\%\).
  3. Its big nuclei can be smashed into fragments; hydrogen nuclei cannot.
  4. Water and trash.

Common mistake

"Lead is the best shield for everything." Lead is good against gamma rays. Against cosmic rays, it makes showers of secondary radiation and is very heavy to launch. Hydrogen-rich materials do better per kilogram.

Where it leads