# Why make X-ray shielding from titanium, when lead is 6 times lighter?

Probes visiting Jupiter and its moons protect their equipment from X-rays with titanium shielding. Juno had 180 kg of the stuff. Europa Clipper is expected to have about that much too.

Terrestrial shielding such as the apron you wear at the dentist uses denser materials such as lead. (Almost anything is denser than titanium!)

Compared to titanium, lead has 4x the atomic weight and 2.5x the density, so per kg you'd expect lead to shield better, because 4 / 2.5 > 1.

This is confirmed by the Health Physics Society:

A titanium [dentist's patient] apron would weigh about 6.6 times a lead apron.

Their calculations use a NIST table of mass attenuation coefficients and mass energy-absorption coefficients as a function of photon energy, 0.001 to 100 MeV. That range covers (I think) most of what Jupiter spews at its moons.

If low mass is so important on a space probe, why is titanium preferred to lead, which would be lighter, cheaper, easier to machine, etc?

• The forms of radiation trapped by Jupiter's magnetic fields will be charged particles, not photons. Depending on the specifics, charged particles are stopped by electron density not mass density, and electrons scale with protons, not protons+neutrons. Titanium has more electrons per kilogram than lead because heavier nuclei have way more neutrons than protons. It's more complicated at high energy (many MeV) where electrons and photons both make "showers" of electrons+photons and pair-production scales with $Z^2$. – uhoh Dec 3 at 2:06
• There’s stopping data for electrons, including plotting ability, in the ESTAR database: physics.nist.gov/PhysRefData/Star/Text/method.html – Bob Jacobsen Dec 3 at 2:59
• The atomic radius of titanium is also slightly smaller than lead, meaning you get more nuclei packed into the same space, and lots of lighter nuclei are better than fewer, heavier ones when stopping big particles (eg. things heftier than electrons). – Starfish Prime Dec 3 at 9:34
• You can find plots of electron vs proton density at Jupiter here: lasp.colorado.edu/home/mop/files/2015/08/jupiter_ch27-1.pdf – Bob Jacobsen Dec 3 at 14:38
• @uhoh I'd accept that as an answer because it corrects what the shielding is for: not X-rays. – Camille Goudeseune Dec 3 at 17:46

From the NASA web site for Juno:

While other materials exist that make good radiation blockers, engineers chose titanium because lead is too soft to withstand the vibrations of launch, and some other materials were too difficult to work with.

There are harder and softer lead alloys, but nothing begins to approach titanium.

Later on the same page:

Each titanium wall measures nearly a square meter (nearly 9 square feet) in area, about 1 centimeter (a third of an inch) in thickness, and 18 kilograms (40 pounds) in mass. This titanium box -- about the size of an SUV's trunk – encloses Juno's command and data handling box (the spacecraft's brain), power and data distribution unit (its heart) and about 20 other electronic assemblies. The whole vault weighs about 200 kilograms (500 pounds).

Panels that large and thin are probably much easier to build with a higher-strength material.

• So why not lead on plywood? See @uhoh comment. – amI Dec 3 at 8:10
• Interestingly, some of the testing process mentioned in that link involved gamma rays rather than electrons. I wonder if that's a proxy for bremmstrahlung caused by electrons hitting the shield? – Starfish Prime Dec 3 at 9:28
• But what about a sandwich made from layers of lead between layers of titanium. – Uwe Dec 3 at 22:38