The home of the SPS

The article by Lyn Evans celebrating 40 years of SPS operation (CERN Courier July/August 2016 p61) raised a lot of memories, but I must take issue with the author on one point. He says that the machine was initially supposed to be built in the south of France, whereas in fact there were five candidate sites advanced by the Member States. The achievement of John Adams in presenting an acceptable – and cheaper – plan for the SPS at CERN was enormous. It was helped by one simple point: CERN was where the skills were, where the people best equipped to design and build the machine lived.

This same argument played a part in the approval of LEP a decade later. Papers had been published that purported to show that the new machine should be built in Hamburg rather than Geneva. This caused problems at the political level in one Member State to my knowledge, and I guess at least two.

• John Walsh, Swindon, UK.

Neutrino discovery date

Christine Sutton’s article about the detection of neutrinos (CERN Courier July/August 2016 pp17–19
) correctly states that Cowan and Reines were the first to see neutrinos at a distance from the emitting nucleus via inverse beta decay.

However, a series of experiments beginning in 1936 (e.g. Proc. Camb Phil. Soc. 32 301) searched for evidence of the neutrino’s existence by measuring the energy and momentum of the recoil of the nucleus and of all other particles emitted in a beta-disintegration decay process. These experiments showed clearly that missing momentum and energy in beta decay required an explanation and that the neutrino hypothesis was the best candidate.

In 1938, Crane and Halpern (Phys. Rev. 53 789) used a chlorine-38 beta source in a cloud chamber to observe the recoiling nucleus and the associated beta particle for individual events. Although the track left by the recoiling nucleus was not long enough to allow an energy measurement, it generated ionisation that was assumed to be proportional to the kinetic energy of the recoil motion. Crane himself did not claim to have detected neutrinos, but stated: “It seems now to have been adequately shown experimentally that there is apparent non-conservation of momentum in the beta decay, and that, quantitatively, the maximum amount of extra momentum found is in satisfactory agreement with that called for either by the neutrino hypothesis, or by much more general theoretical arguments which relate the disappearance of momentum to the disappearance of energy.”

• Andrew Sabersky, Chico, California.