## PULSE FORMATION

When the detector is exposed to neutron flux, fission is induced in the uranium coating, U235^, F) fp. Two energetic, highly-charged fission particles travel in opposite directions from their original site. Since the uranium coating is extremely thin, it is highly probable that one of the fission particles will traverse the gas gap in the chamber.

The shape of both the ionic and electronic current pulses from a well saturated chamber have the form:

where

PQ = the charge density along the fission particle track, d = electrode spacing,

t = time from instant of track formation, and

Te = transit time of electrons (or ions) across gas gap.

The typical charged particle track is one that extends across the entire gas gap in the chamber since the electrode spacing is small relative to the range of the fission particle. After passage of a fission particle, the resultant ions and electrons drift respectively toward the electrode of opposite polarity. .

Assume that the rise of the pulse is instantaneous since it is associated with the transit time of the fission particle and the time required for the charged particle to reach terminal velocity. Then, the extent in space of the electrons due to the passage of a single charged particle at time, t, after pulse initiation is as shown in Figure 4-1.

An energy balance equation is written relating the work done by the battery or power supply to work done on gas by the movement of the charged particles. The work required to collect the remainder of the pulse is

[0(d) -0(x)] PQdx = Vp q, (4-2)

where Vp is the battery potential and q is the charge yet to flow at time, t.

But

and

where pe is the electron mobility (assumed to be a linear function of electric field). Therefore,

The charge collected, q’, is given as

At t = o, q’ =o, hence

Differentiate q’ with respect to time to obtain the instantaneous current:

„ VP "e* V‘ e d d3

V / V t

= P0 ^ f f 1 —

The total time interval of the pulse due to the electrons is equal to the transit time of the electrons and is given by

Substitution into Equation (4-8) yields

which is the expression for the current pulse. The time interval of the pulse was measured to be 0.1 дsec.