LEVEL 3 LABORATORY

High Energy Physics

General Electronics in use in the Level 3 HEP Laboratory

Most of the electronics in the laboratory is based upon the internationally accepted NIM standard. The system includes discriminators, coincidence units, amplifiers and scalars. The majority of these units having been designed and built within the department. The department utilises a reduced NIM system allowing the insertion of 8 units in each bin, as opposed to the standard 12. There are also limitations to the positions in which the units can be accomodated into the laboratory’s reduced NIM system. Before any unit is used, be it commercial or department made, it is important that students make themselves familiar with the general characteristics of the unit and its mode of operation. It can not be emphasised too greatly that electronic apparatus will not stand abuse, either physical or electrical. Knobs and switches should be altered gently, the units must not be forced into position and plugs and sockets should be joined carefully. Thought should always be given to the hazards of applying high voltages to particle detectors and to expensive electronic apparatus and the dangers of disconnecting cables which may control potentials in a unit.

The electronics

In this discussion there are two kinds of pulses described: linear pulses and logic pulses. A linear pulse has the size of the pulse proportional to the energy of the event that caused it, such as that from a photomultiplier or amplifier. A logic pulse is a pulse of one size only, such as TTL pulses and the output pulse from a discriminator.

• Pre-amplifiers
  Radiation detectors generally measure an event as a deposition of charge and output this as a measurable signal. A pre-amplifier takes this charge signal and converts it to a voltage, so it can be passed to any other electronics needed for the recording of events.
• Amplifiers
  Amplifiers take the signals from a detector or pre-amplifier and amplify’s them to a usable signal for discriminators or counting equipment. The amplification itself can be a minor role for an amplifier: they can also act as a buffer to the detection electronics; match impedances; and shape the output pulse for timing experiments. For a discussion on the choice of amplifiers for nuclear experiments click on this link
• Coincident units
  A coincidence unit is used in timing experiments. The unit takes a number of input signals and looks at whether they occur in the same time window.
• Discriminators
  Discriminators give out a `standard’ sized pulse (a logic pulse) whenever the input pulse (a linear pulse) exceeds a certain threshold. The unit can be designed to trigger either on the initial crossing of the threshold, or on the second crossing (ie on the falling edge of the pulse). Always check the unit if the timing of the output pulses is important.
• Scalars
  A scalar simply counts the number of logic pulses it receives to its input channel.

Useful Links
The following companies manufacture equipment of the type used in laboratories and their websites contain detailed information on how they work:

• Canberra
• Ortec