3.5.5.5. Deadtime

The time between X-ray entry and electron-ion pair avalanche production is on the order of 3 x 10^-7 seconds. The avalanche produces a sharp voltage drop that decays back to zero relatively slowly taking about 10^-4 seconds. The rate-limiting process in this decay is the slow movement of Ar ions to the cathode tube (10³-10⁴ times slower than electron movement). This slow neutralization of the Ar ions prevents the production of another electron-ion avalanche and produces what is called a deadtime, an interval during which the detector is inoperative and fails to detect incoming X-rays.

The methane in P-10 detector gas is an electron donor and reduces the deadtime by providing "quench" electrons to the Ar ions. These help halt the avalanche of electron-ion pairs because the neutralization of the Ar ions is not dependent on Ar ions actually reaching the cathode tube. The quench gas allows the tube to reset itself faster decreasing deadtime. In addition, the electronics are often set to "clip" the pulse after about 10^-6 seconds. Deadtime is routinely about 1 to 2 µsec. Deadtime becomes very important at count rates exceeding 3000 cps (Figure 3.5.5.5).

Figure 3.5.5.5. Plot of output count rate as a function of input count rate, for four deadtime constants.

The effect of detector deadtime can be corrected using the formula:

where I_m = measured counts, I_t = true counts, and t_d = deadtime.

For example, with t_d = 2 x 10^-6 seconds and I_m = 10⁵, I_t = 1.25 x 10⁵ (25% higher). Deadtime corrections are routinely made by the microprobe automation system. Deadtime can be determined by accumulating a series of intensities, I, at different beam currents, i.

For any current, the intensity is proportional to the current:

Thus we can rewrite the deadtime equation:

Plotting I_m/i against I_m will yield a straight line and the value K can be determined by extrapolating the line to I_m = 0. Once K is known t_d can be obtained from any point on the line by:

Values calculated from higher values of I_m will generally be more accurate because of better counting statistics.