Pulse-counting detectors

The experimental apparatus consists of eight main parts an ultraviolet flashlamp capable of repetitive flashing at about 5 Hz, a purge flow reactor with either pinhole or molecular beam sampling, an ion source, a mass filter, an ion detector, pulse-counting electronics, computer data aquisition, and a vacuum system. A diagram of the apparatus is shown in Figure 1. 

A scintillation ion detector, described in detail elsewhere (41), detected virtually every ion which entered the detector chamber. Pulse counting techniques were used. 

Detection systems. Prior to the past decade, most instruments used for uranium-series analysis were single-collector instruments, for which ion beams of the various isotopes are cycled onto a single low-intensity detector, usually with electronics operating in pulse counting mode (Chen et al. 1986 Edwards et al. 1987 Bard et al. 1990 Goldstein et al. 1989 Volpe et al. 1991 Pickett et al. 1994), in order to measure the low-intensity ion beams of °Th, Pa, Pa, Ra and Ra. Daly detectors and... 

Quantitative Analysis. The efficiency of the detector is such that almost 100% of the X-rays entering it will produce a pulse, but the pulse processing speed limits the rate at which X-rays can be counted. If the count rate is less than a few thousand counts per second, then most of the incoming pulses are processed, but as the count rate rises an increasing fraction of the pulses are rejected. The live time during an analysis when the detector was counting is thus less than the elapsed time, and the EDS system records both times in order that the true count rate may be measured. 

In the measurement technique, which has been used on D3 for many years, the ratio of the time spent counting with the cryoflipper in (+) or (-) mode is controlled by a quartz crystal controlled oscillator with a highly stable output frequency / of 1 MHz. There are two scalers to count the detector pulses (+ and - states), a single monitor scaler and a single time scaler used to end the measurement when the total time is reached (precision of 1 ms). 

Fig. 8. The relative uncertainty in the measured intensity of various detectors as a function of the exposure level. A solid straight line indicates an ideal detector. A dashed line indicates a pulse-counting detector of 10 % efficiency. O and indicate the IP system for MoKa and CuKp, respectively. A and A indicate Kodak DEF-5 X-ray film for MoKa and CuKp, respectively. The munber of X-ray photons required to obtain a certain accuracy in intensity measurements can be compared...

Another commonly used detector is the Faraday cup. This detector is an analogue detector and so has poorer sensitivity than a pulse counting electron multiplier. However, it has the advantage of simplicity (it is essentially only a metal plate used to measure ion current), and it does not suffer from burn-out like an electron multiplier (which must be periodically replaced). 

Also important is the effect of detector dead time. When ions are detected using a pulse counting (PC) detector, the resultant electronic pulses are approximately 10 ns long. During and after each pulse there is a period of time during which the detector is effectively dead (i.e. it cannot detect any ions). The dead time is made up of the time for each pulse and recovery time for the detector and associated electronics. Typical dead times vary between 20 and 100 ns. If dead time is not taken into account there will be an apparent reduction in the number of pulses at high count rates, which would cause an inaccuracy in the measurement of isotope ratios when abundances differ markedly. However, a correction can be applied as follows ... 

Electron multipliers can also be operated in analog mode as current detectors. In this mode, they have a lower gain and measure higher signals than in pulse-counting mode. This... 

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

These saturation effects, resulting from the loss of ions due to the TDC dead time, can be statistically corrected by applying a correction factor [4], However, there is no effective correction when the quantity of ions increases and several ions arrive at the detector simultaneously. In conclusion, detection systems that operate in pulse counting mode are well suited to detect small quantities of ions by accumulation for a long period of time and when detection of individual ion events is important in order to obtain a good signal-to-noise ratio. 

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