Introduction

In the context of GEANT:

• hit is the user-defined information recorded at tracking time to keep track of the interaction between one particle and a given detector, and necessary to compute the digitisations later.
• digitisation ( digit) is the user-defined information simulating the response of a given detector element, usually estimated after tracking a complete event.

For complex setups with different types of detectors the user has normally to define several types of hits and digitisations. In addition to the hits generated by all particles of the current event, computing the digitisations requires usually some information about the intrinsic characteristics and performance of the detectors. The information to be recorded for the hits and digitisations is highly experiment dependent, therefore only a framework can be proposed to store it.

Two remarks can be made:

• during the life of an experiment, the stability of the format and content of the information to be stored is usually reached much earlier for the hits than for the digitisations. Therefore the user may save computing time by designing an intermediate event output at the hits level.
• the scheme proposed for storing the digitisations should in any case be considered as an intermediate stage, a further processing of the data being necessary if the user wants to simulate more closely the specific format of the real data-acquisition system.