Out with the old, in with new?

21 March 2011

Professor Peter Hobson of Brunel University intrigued his audience on 16 March by starting his lecture with what appeared to be fashion shots.

IOP event

Professionally lit elegant models, male and female, sported books on one side of the screen and e-books on the other. 

This was followed by a sequence of pictures of redundant bits of equipment: a very large bubble chamber at CERN; a triode radio valve and some old household items: a curvaceous Bakelite radio; an equally rounded television with a cathode ray tube (CRT) screen; a telephone with a dial and a heavy cut-glass decanter. What had these to do with the Large Hadron Collider (LHC) and all its electronic and solid state gadgetry?

The answer was the return of some particle detectors thought to have been superseded, like the bubble chamber, which made its re-entry in 2010, thanks to the discovery the acoustic signals triggered by particles passing through are characteristic of each type, so that an α-particle “sounds” different from any other.

The Compact Muon Solenoid Detector (CMS) of the LHC contains 100 tons of lead tungstate crystals placed around the collision site to measure the energy of particles emerging from collisions. Professor Hobson is holding up one of the crystals in the second photograph with Barbara Gabrys, Chair of the London and South East Branch looking on. 

Scintillations of light created within each of the hundreds of crystals are detected at the far end by a vacuum phototriode, looking rather like that radio valve, except that the cathode is photovoltaic not thermionic. What is more, it works even in the 4 Tesla magnetic field needed to determine the charge and momentum of each piece of debris. 

Interestingly, lead tungstate was rejected as a phosphor for CRTs in televisions, because its efficiency is very low, but that does not matter at the high collision energies in the LHC and it is not prone to radiation damage.

Professor Hobson pointed out that, while the lead glass of the decanter has been essential for the Cherenkov detectors in previous experiments at CERN, it would blacken under intense radiation in the LHC and so could not serve as windows to allowing viewing. However, large glass scintillators may come back as detectors of the heavier hadrons.

The oldest and cheapest plastic of the radio case and the telephone, Bakelite, has made a come back in the form of large scale resistive plate counters (RPC), which we could see in another picture radiating out from the ends of the axis of the CMS Detector.

The meeting ended with some lively discussions of other older technologies: cloud chambers and photographic nuclear emulsions and then another round of applause and agreement that the older technologies should not be forgotten, because they may be useful, perhaps in another guise. 

Afterwards, as shown in the picture, members of the audience crowded round to see and handle some of the materials seen in the lecture.