Physics and Engineering of Radiation Detection

12.5. Time Spectroscopy 711

The incident particle of choice for mass spectroscopy is the electron due to several
reasons. First of all, it is a fundamental particle and does not fragment itself during
the collision. Also its small mass makes it easier to be accelerated in a small scale
accelerator. A highly intense beam of electrons is therefore easy to produce as
compared to other heavier particles, which require big accelerators.
A typical mass spectrometer is shown in Fig.12.4.1. The sample is first vapor-
ized by a heater and then bombarded by a beam of electrons. Impact of electrons
dissociates the sample into small fragments. The positively charged fragments are
accelerated towards a powerful magnet through electrodes that are kept at high
electrical potentials. Upon their passage through the magnet, the fragments flay
apart from each other. The amount of deflection is proportional to their mass to
charge ratio. However since most of the fragments have unit positive charge, their
separations are proportional to their masses. The result is the production of several
beams of like-mass particles, which are detected by a position sensitive detector. In
the earlier days of mass spectroscopy, photographic films were used to detect the
particles. With the advent of easily available large area position sensitive electronic
detectors, the use of photographic films has faded away.

Accelerating

Electrodes

Detector

Heater

Electrons Sample

Vaporized

Vacuumed

Particle Path

Positively Charged

Fragments

Magnet

Figure 12.4.1: Working principle of a mass spectrometer.

12.5TimeSpectroscopy

In time spectroscopy one is interested in determining the relationship between the
arrival times of particles emitted as a result of some event. Note that here we are
not talking about coincidence timing, rather difference in the arrival time of pulses.
However a time spectroscopy system can as well be used for coincidence spectroscopy.
Any detector having sufficient time resolution and response time can be used
to build time spectrometer. In most cases, a combination of scintillator and pho-
tomultiplier tube (PMT) is used. A typical time spectrometry setup is shown in
Fig.12.5.1. The two detectors individually produce pulses as a result of passage of
radiation. These pulses are made to pass through two so calledtime pickoffcircuits.
A pickoff circuit produces a sharp logic pulse as soon as the input pulse amplitude