Pulse amplifier

The most basic elements in a liquid scintillation counter are the PMT, a pulse amplifier, and a counter, called a scaler. This simple assembly may be used for counting however, there are many problems and disadvantages with this setup. Many of the difficulties can be alleviated by more sophisticated instrumental features. Some of the problems and practical solutions are outlined below. 

The pulses are provided by a precision bipolar voltage source, which is switched into the input of the pulsing amplifier by the switch at point A in the circuit. A very accurate crystal-controlled timing circuit (not shown) drives the switch to ensure that the pulses are symmetrical. The pulsing amplifier inverts the signal as shown by waveform B and supplies current to the cell. The cell current is amplified by the current follower, the output of which is illustrated by waveform C. 

Linear amplifier Linear tail pulse Amplified and shaped linear pulse... 

The results of these experiments were in reasonable agreement although the Stanford group reported rather smaller errors. Subsequent, continuous-wave experiments at both Stanford and Oxford show poor agreement with the Stanford pulsed result. This has led to speculation that the frequency chirp in pulse amplified experiments is so difficult to characterise that pulse amplifiers cannot be used for precision measurements. 

We have undertaken an experiment to try to improve the performance of pulse amplifier experiments. The system is shown schematically in figure 2. It consisted of a continuous-wave C102 dye laser amplified in three stages by a frequency tripled Q-switched NdtYAG laser. The output energy was approximately 2.0 mJ in a 150 MHz linewidth and was up-shifted from the continuous-wave laser by 60 MHz caused by the frequency chirp. This light was then spectrally filtered in a confocal interferometer with a finesse of 40 and a free spectral range of 300 MHz. The linewidth of the filtered radiation was approximately 16 MHz. 

Fig 3 Spectrum of the hydrogen IS to 2S transition taken using pulse amplified radiation. 

Fig 2 Schematic diagram of the apparatus used to observe the IS to 2S transition in hydrogen using a pulse amplified laser. 

Figure 6 Block diagram of the two-color optical parametric amplifier (OPA) and IR-Raman apparatus. CPA = Chirped pulse amplification system Fs OSC = femtosecond Ti sapphire oscillator Stretch = pulse stretcher Regen = regenerative pulse amplifier SHGYAG = intracavity frequency-doubled Q-switched Nd YAG laser YAG = diode-pumped, single longitudinal mode, Q-switched Nd YAG laser KTA = potassium titanyl arsenate crystals BBO = /J-barium borate crystal PMT = photomultiplier tube HNF = holographic notch filter IF = narrow-band interference filter CCD = charge-coupled device optical array detector. (From Ref. 96.)...

Such single-mode lasers, often pulse amplified by dye laser amplifiers pumped by injection-locked Nd YAG lasers, are used in nonlinear Raman techniques by which an instrumental resolution better than 0.001 cm is achieved (Esherick and Owyoung (1982), Schrotter et al. (1988a)). 

Pulse amplifiers with gain greater than 6 decibels and with a baseband bandwidth greater than 500 megahertz (having the low frequency half-power point at less than... 

Figure 2. ARC AS deck unit showing filter transport mechanism (A), aspirating motor (B), motor controllers (C), radiation-counting photomultiplier tid)es (D), pulse amplifier transmitter units (E), high- and low-voltage power supplies (F), calibration counter (G), filter supply roll (H), and takeup roll (I).

The absorption of a quantum (photon) of x-rays in the active volume of a counter causes a voltage pulse in the counter output. Pulses from the counter then enter some very complex electronic circuitry, consisting of one or more pulse amplifiers, pulse shapers, etc. and, at the end, a scaler or ratemeter and, possibly, a pulse-height analyzer (Sec. 7-9). Let us call all the circuitry beyond the counter simply the electronics. Then we are interested not simply in the behavior of the counter alone, but in the behavior of the whole system, namely, the counterelectronics combination. 

The counting rate varies linearly with x-ray intensity up to rates of about 5,000-10,000 cps. Counting losses in the counter-electronics system occur in the electronics rather than the counter. The electronics are more complex than usual and include, besides the usual pulse amplifiers and shapers, a multichannel pulse-height analyzer (Sec. 7-9). 

Fig. 127. Flame laser enhanced ionization spectrometry [670, 671]. (a) Flashlamp/dye laser, (b) high voltage, (c) trigger photodiode, (d) preamplifier, (e) pulse amplifier, (f) active filter, (g) boxcar averager, (h) chart recorder. (Reprinted with permission from Ref. [671]).

In the experiments described here, two separate techniques have been used for interferometric characterization of the shocked material s motion frequency domain interferometry (FDI) [69, 80-81] and ultrafast 2-d spatial interferometric microscopy [82-83]. Frequency domain interferometry was used predominantly in our early experiments designed to measure free surface velocity rise times [70-71]. The present workhorse in the chemical reaction studies presented below is ultrafast interferometric microscopy [82], This method can be schematically represented as in Figure 6. A portion of the 800 nm compressed spectrally-modified pulse from the seeded, chirped pulse amplified Ti sapphire laser system (Spectra Physics) was used to perform interferometry. The remainder of this compressed pulse drives the optical parametric amplifier used to generate tunable fs infrared pulses (see below). 

The error is a systematic error arising from ac Stark effect, second order Doppler shift, a frequency offset between the cw dye laser oscillator and the high-powered pulsed amplifier output and laser metrology. Substantial improvement in the accuracy of this measurement appears technically feasible. The current theoretical value is(6,15)... 

An alternate approach used by the Miller group is to pulse amplify a CW dye laser. This approach degrades the laser resolution from about 1 MHz to about 200 MHz but results in a high-power pulsed laser beam with excellent mode quality. This high peak power and high spectral resolution results in excellent spectra with an improved S/N ratio. The only serious drawback of this scheme is the increased complexity and cost of the laser systems. 

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