Question

Brieflydiscuss:
(i) The operational principals and the main characteristics of photomultiplier tubes

(PM). Why does a PM tube still have an output signal even in absolutely dark

conditions? What is affecting timing of the signal in a PM tube?
(ii) The response functions of 'small', 'middle' and 'large' gamma-ray spectrometers.

(iv) Spectroscopy setup and output signals from each part of setup.

Answer 1

Operational principles of PMT is just based on photoelectric effect. Visible light incident on photocathode, produces electrons, which are accelerated by electric field and then multiple photoelectric events multiply the no. Of electrons, producing a signal worth recording for the electronics.

Gamma rays entering the Scintillator result in a signal of PMT.

As Gamma rays donot fall in visible spectrum of light, even in dark room, PMT responds.

Otherwise if no source is present, our environment has Gamma rays coming from outer space, it may respond to that though feebly.

2.

Small detectors miss events, photopeak is observed, Compton is observed but for pair production double escape and single escape peaks may be seen, 2*0.511MeV less than photopeak.

Large detectors may show pile up of all signals and therefore just photopeak as all interaction funallf lead to photoelectric effect.