Cosmic rays, pyramids, volcanoes and railway tunnels — an introduction to muon tomography.    Prof Lee Thompson. 16th March 2020 

Muons are like the big brother of elec­trons but they are heav­ier and much more pen­et­rat­ing.  About 1000 pass through the room every second. They are gen­er­ated when cosmic rays inter­act with atoms in the upper atmo­sphere to form sec­ond­ary rays called muons.

We all know that X-rays are used to look at body parts that are hidden in soft tissue.

  • A beam of X-rays is focused on an object
  • A detector (X-ray film) is placed on the other side of the object
  • Density dif­fer­ences between soft tis­sues and bones etc. enable the interior to be seen

Muon tomo­graphy (ima­ging with muons) uses an identical method (but muons are free and do not need expens­ive equip­ment to gen­er­ate them!)

Lee did not give details of the muon detector but said  that muons come from above and also some came from the sides.  They are even­tu­ally all absorbed by the soil and rocks until sev­eral hun­dred metres below the sur­face.

Tomography has been used to find hidden cham­bers in the Great Pyramid.

Muons coming from the side have been used to image magma cham­bers in vol­ca­noes. In this case images of the top of the magma column before and after an erup­tion were used to pre­dict fur­ther erup­tions.

On the rail­ways in the U.K. there are many tun­nels dating back to the Victorians. Often tun­nels were made by sink­ing shafts to the depth of where the tunnel was to be made. Then  crews dug hori­zont­ally in both dir­ec­tions until they met up with other sim­ilar parts of the tunnel.  In many cases these shafts were filled in (with rubble) or some were left open as air shafts.  Some of the shafts are not stable now and some tunnel roofs are in danger of col­lapse.  However, very few records of where these shafts were dug are avail­able.

The rail com­pan­ies have a pro­gramme to avoid more col­lapses but they cannot close tun­nels that are in use.  They send in teams of drillers on scaf­fold­ing to drill into the roofs of tun­nels.  This is a prob­lem because it takes lots of men, lots of time, only a few areas can be examined and the drilling may weaken the roofs.

Lee’s work is aimed at find­ing hidden shafts using muon tomo­graphy. British Rail is allow­ing his team to use muon tomo­graphy in Alfreton Old Tunnel, which is 760 metres long and dis­used.

The team have built a muon detector small enough to fit into a Transit van. They moved the van into the tunnel and took read­ings for 30 minutes then moved on 5 metres and took more read­ings for 30 minutes and so on.

The graph of the read­ings showed high read­ings before they entered the tunnel  then fell off at the tunnel entrance as the over­bur­den above the tunnel absorbed the muons.

The graph showed steady low read­ings for a length of tunnel then the read­ings rose slightly at one spot then dropped back for a length and then rose again and so on.

The rail com­pany had told Lee’s team where two shafts were but they found three areas where the read­ing rose.  Two were at the areas they expec­ted, but they found three pos­sible shafts.  The com­pany then admit­ted that they knew of the third shaft  80 metres into the tunnel, but they were test­ing the team to be sure their res­ults were genu­ine.

Lee’s team went back and did another more detailed survey of this tunnel and found that other “hot spots” gave inform­a­tion about shafts with only 5 metres of over­bur­den sug­gest­ing more part-filled, unknown shafts.

This method needs cooper­a­tion from a geo­lo­gical survey team. It is obvi­ous that,  if the sur­face of the over­bur­den land is not flat, then the over­bur­den will be thicker in some parts than others and will absorb more muons.  This has to be allowed for when inter­pret­ing the tomo­graphy res­ults.

Muon tomo­graphy can be used to detect smuggled nuc­lear mater­ial.  If a detector is placed under and around the sides of sus­pec­ted vehicles then uranium and plutonium will deflect muons but light mater­i­als will not and this will show up on the res­ults.

Nuclear waste from 1950 -60 is poorly doc­u­mented.  Canisters can be checked by muon tomo­graphy to find if they are full or if they have leaked.

There may be many other uses for muon tomo­graphy that will be developed in the future.

Lee’s team are still work­ing on it!

This was an extremely inter­est­ing talk for all, but espe­cially for this ex-Chemistry teacher.