Photomultiplier Transit time

L. E. Wood, T.K. Gray, M.C. Thompson Technique for the measurement of photomultiplier transit time variation. Appl. Opt. 8, 2143 (1969)... 

The transit times of photoelectrons in normal photomultipliers depend significantly on the wavelength of the incident photon, whereas this dependence is... 

Low transit-time dispersion with photon wavelength, i.e., < 0.5 psec/nm. This minimizes the effect on convolution of the difference between the excitation and fluorescence wavelengths. Both side-window and linear focused photomultipliers satisfy this. 

Low transit-time dispersion with point of illumination on the photocathode, i.e., < 20 psec/mm. Linear focused photomultipliers satisfy this criterion, but side-window devices do not. This again is relevant to successful data analysis. 

The operating principle of an MCP-PM is based on electron multiplication using a continuous dynode structure of ca. 10 um diameter holes, giving a more compact and hence faster time response when compared with conventional photomultipliers. Rise-times of 150 psec and transit-time jitter (i.e., impulse response) of ca. 25 psec FWHM at 200 counts/sec noise at room temperature have been recorded with the 6 fun channel Hamamatsu R3809 MCP-PM.(87)... 

When time-dependent signals are to be measured by a photomultiplier, the time sensitivity is usually limited by the inhomogeneous transit time. The transit time is the time taken by electrons generated in the cathode to arrive at the anode. If all of the emitted electrons had the same transit time, then the current induced in the anode would display the same time dependence as the incoming light, but delayed in time. However, not all of the electrons have the same transit time. This produces some uncertainty in the time taken by electrons to arrive at the anode. There are two main causes of this dispersion ... 

Figure 3.20 shows the effect of the transit time dispersion on the measurement of an ideal light pulse. Since photoelectrons spend some time traveling from the photocathode to the anode (transit time), the photomultiplier signal is delayed in time with respect to the incident pulse. Furthermore, due to the transit time dispersion, the... 

The transit time broadening has been further reduced by installing a small aperture in front of the photomultiplier. Thereby, only atoms that travel close to the axis of the enhancement cavity contribute to the signal. 

The time resolution of the electronics in a single photon counting system can be better than 50 ps. A problem arises because of the inherent dispersion in electron transit times in the photomultiplier used to detect fluorescence, which are typically 0.1—0.5 ns. Although this does not preclude measurements of sub-nanosecond lifetimes, the lifetimes must be deconvoluted from the decay profile by mathematical methods [50, 51]. The effects of the laser pulsewidth and the instrument resolution combine to give an overall system response, L(f). This can be determined experimentally by observing the profile of scattered light from the excitation source. If the true fluorescence profile is given by F(f) then the... 

In evaluating the experimental data obtained from the time-resolved measurements we have taken into account artificial time lags of different origins, such as the electron transit time of the photomultiplier. The experimental data presented here was corrected for these time lags. 

The Na source is placed between two identical samples. Two XP 2020 photomultipliers equipped with scintillators are attached directly to the two samples. The pulses from the photomultipliers are used as start and stop pulses for the TCSPC module. The pulses from PMT 2 are delayed by a few nanoseconds so that a stop pulse arrives after the corresponding start pulse. Eaeh y quantum generates a large number of photons in the scintillator. Therefore, the PMT pulses are multiphoton signals, and the time resolution can be better than the transit time spread of the PMTs. Moreover, the amplitudes of the photomultiplier pulses are proportional to the energy of the particle that caused the scintillation. Therefore the amplitudes can be used to distinguish between the 511 keV events of the positron decay and the 1.27 MeV events from the Na. The discriminator thresholds for start and stop are adjusted in a way that the stop channel sees all, the start channel only the larger Na events. The rate of the Na events is of the order of a few kHz or below. 

Table 4.1. Transit Time Spreads (TTS) of Conventional and MicroChannel Plate (MCP) Photomultiplier Tubes (PMTs) ...

The present limitations in time resolution for the time-correlated photon counting technique are due to the time jitter in the detection dectronics and the transit time read in the photomultiplier tube ( SOO ps). Wth future improvements in these components and using cwmode4ocked lasers as an excitation source, deconvolution of fluorescence lifetimes to a few tens of picoseconds riiould be achieved. Alter-... 

The time resolution of a photomultiplier is limited mainly by the variations in the paths that electrons take in reaching the anode. Because of the spread in transit times, the anode pulse resulting from the absorptitm of a single photon typically has a width on the order of 10 -10 s. The spread of transit times is smaller in microchannel plate photomultipliers, which work on the same principles as ordinary photomultipliers except that the electronic amplification steps occur along the walls of small capillaries. The anode pulse width in a microchannel plate detector can be as short as 2 x 10 s. 

Fig.11.6a,b. Observed output pulses from an argon laser at X = 488 nm, actively mode locked by an acousto-optic modulator, (a) Detected with a fast photodiode and a sampling oscilloscope, (b) detected by single-photon counting technique using a photomultiplier. The oscillations following the optical pulse in (a) are due to cable reflections of the electric output signal from the diode. The pulse width in (b) is limited by electron transit time variations in the photomultiplier [11.10]... 

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