CHAPTER 3
INSTRUMENTATION
 
 
    1. Discriminator

    2. [Testing]
      Discriminator is used to eliminate the low frequency signals. In this system we use ORTEC Model 436 100 MHz Discriminator. Figure 3.1 describes discriminator's threshold effect on count rate. When discriminator threshold is high, the noise will be filtered and we can get zero noise count and pulse signal passing through discriminator, on the contrary, when threshold is low, the noise count will be high, the output will be the mixture of pulse signal and noise.

      Fig. 3.1 Effect of Discriminator Threshold
      on Signal Amplitude

       

    3. Amplifier
 

The amplifier is one of the most important components in a pulse processing system for applications in counting, timing or pulse-amplitude spectroscopy. Normally, it is the amplifier that provides the pulse-shaping controls needed to optimize the performance of the analog electronics. EG&G Model AN201/N Quad Amplifier is used to amplify signal coming from photomultiplier tube. This amplifier provides a compact and cost-effective solution for experiments where timing is required on a number of detectors.

3.3 Scaler

A scaler counts the number of events that occur during the time interval t to t+D t as a function of time. ORTEC Model 430 Scaler is a high-speed, six decade scaler which has a printing output and a built in discriminator. It provides front panel switch selection of Master, Slave or Normal operation mode. Any scaler can be switched as master to control the counting of all slave units in the system or switched to on/off counting control or signal-to-gate input. The 430 contains a direct-coupled internal discriminator for input signals from 0.1v to 11v.

 

3.4 Timer

Timers count pulses generated by an internal clock and are used to measure elapsed time or to establish the length of counting period. EG&G ORTEC timers contain preset controls to establish the duration of the counting period. When counting is initiated, the internal clock pulses are counted until the preset condition is reached, at that time, counting is stopped in all counters connected to the common gate line of the master timer. If the external input is used, the preset control will apply to counting of the pulses at the external input and will result in preset count operation.

    1. Power Supply
    2. This system uses Model 1 HV-1544 High Voltage Regulated DC Power Supply by Power Designs Pacific INC, PALO ALTO California. It has 1-3000v DC, 20 MA range. The voltage has been set to working at 1950v. The polarity is on negative.

    3. Delay Box
    4. The function of delay box is to align fast-timing channels that incorporate coincidence circuits or time-to-amplitude converters. ORTEC Model DB-263 delay box is used in this experiment. It provides relative delays from 0 to 63.5 ns with 0.5-ns increments in each of four identical sections. Longer delays may be achieved by cascading several delay box sections. We can use delay box to adjust the signals to make them to come at the same time when there exists time shift between signals.

      In experiments involving several sources of analog and logic signals, the signals from different paths usually must be aligned to arrive simultaneously at the decision points. This is the function of delay modules. Coaxial cables or lumped-parameter delay lines are used to generate the delay.

      Because of the nature of Time-to -Amplitude (TAC) circuitry, it is difficult to measure time intervals <10 ns with good linearity. However, many measurements involve start and stop signals that arrive within ± 10 ns of each other. The solution for these situations is to insert an appropriate delay in stop signal path. Selecting a delay in the range of 10-30 ns on an EG&G ORTEC nanosecond delay box is sufficient to move the timing peak into the linear region of the time spectrum. The stop delay can also be adjusted to center the features of interest in the time spectrum. Figure 3.2 provides some idea of how inputs A and input B overlap.

      C=A· B

      Fig.3.2 The Basic Principle of
      Overlap Coincidence Circuit

      The overlap coincidence circuit is a two-input AND gate. The AND gate generates a "logic 1" output only when "logic 1" pulses are present on both A and B inputs. The output is generated only for the time during which A and B pulses overlap.

    5. Time To Pulse Height Converter
    6. Time-to-Amplitude Converter (TAC)

      ORTEC Model 457 Biased Time to Pulse Height Converter also known as Time-to-Amplitude Converter (TAC),is used in the system. It provides an output pulse with an amplitude proportional to the time interval between a start and subsequent stop pulse input. The input pulses are supplied by a fast discriminator, or may come from the anode of a photomuliplier tube or similar device such as a light-sensitive diode, etc.

      The Model 457 has 15 time ranges from 50 nanoseconds to 80 m s. A time resolution of 10 picoseconds FWHM is possible on the 50-nanoseconds range.

      The time-to-amplitude conversion signal is generated only after a start signal has initiated the conversion process and a stop signal has been received within the selected time range. No output signals are generated by any start or stop signals outside the selected time range. Therefore, any start signal that is not accompanied by a stop signal within the selected range will produce no output signals. The author measured delay time and output signal amplitude relationship in laboratory using an oscilloscope. The relationship can be seen from the spreadsheet chart below: It is a linear relationship.

       

       

       

       

       

       

       

       

      Fig.3.3 Pulse Amplitude / Delay Time Relationship

       

      A high resolution TAC measured the time interval from the first accepted start pulse to the next stop pulse. It ignores all subsequent start pulses and any additional stop pulses until it has finished converting the first pair of start and stop pulses. If either input is receiving randomly distributed pulses at a very high counting rate, the TAC will prefer to analyze the pulses that arriving earlier on that input and well suppress the pulses that arrive later. This will distort the measured time spectrum for correlated start and stop events. The distortion can be controlled by limiting the counting rates at the start and stop inputs. From Poisson statistics [21], it can be shown that limiting the average random counting rate R at both start and stop inputs to

      R<0.01/Trange

      will ensure the number of suppressed pulses in the analyzed time range Trange to be less than 0.5% of the number of accepted pulses on the respective input. This condition will adequately ensure the distortion of the time spectrum less than 1%.

      For a short time range, Trange =50ns, the condition above limits the counting rate to 200,000 counts/s at both start and stop inputs to the TAC. This counting rate is still high enough to require an Multichannel Analyzer with a conversion time of 5m s or less in order to keep up with the data from the TAC.

       
       

      Biased amplifier of Model 457 and coarse and fine gain are used to allow a region-of-interest in the time range to be expanded and examined in detail. The gain of the biased amplifier is used to magnify the time scale. The stability of the gain and bias levels prevents degradation of the time-to-amplitude conversion process and preserves the excellent time resolution.

       

    7. Multichannel Analyzer
    8.  

      Multichannel analyzer (MCA) is software-controlled instrument to provide complete flexibility in data acquisition. The multichannel analyzer (MCA) consists of an analog-to-digital converter (ADC), a histogramming memory, and a visual display of the histogram recorded in the memory. The purpose of ADC is to measure the maximum amplitude of an analog pulse and convert that value to a digital number. This digital output is a proportional representation of the analog amplitude at the ADC input. For sequentially arriving pulses, the digital outputs from the ADC are fed to a dedicated memory and sorted into a histogram to record the number of events counted in each pulse-height interval. This histogram represents the spectrum of input pulse heights. If the input pulses come from an energy spectroscopy amplifier, the histogram corresponds to the energy spectrum observed by the associated detector. When the output of a time-to-amplitude converter is connected to the ADC input, the histogram represents the time spectrum measured by the time-to-amplitude converter. The combination of the ADC, the histogramming memory, and a display of the histogram forms a

      multichannel analyzer. A computer is employed to display the spectrum, it is called multichannel buffer (MCB).

      Fig.3.4 Functional Block Diagram of

      Multichannel Analyzer

      In our system, EG&G ORTEC 916 MCB card and ACE4-B1 Multichannel Analyzer are used to store and evaluate counting data.

       

    9. Photomultiplier Tube
Photomultiplier Tubes (PMTS) are light detectors that are useful in low intensity applications such as fluorescence spectroscopy and biophoton detection. Due to high internal gain, PMTs are very sensitive detectors. A photon enters the photomultiplier through the glass surface and hits the photo-cathode, which is placed on the inner surface of the glass. The photo-cathode hit by the photon emits an electron. A photo-cathode is just a special substance which is very efficient at doing this. Inside the photomultiplier there is a vacuum.

The electron is attracted and accelerated to the first dynode, which is charged positively by a high voltage. As it hits it with great energy, the dynode emits several electrons, which are then attracted to a second dynode, which has even higher positive electric potential. This process repeats many times. At the last dynode we have a

really huge number of electrons. This way the signal of a

single electron was enormously amplified. This is why photomultipliers are such sensitive light detectors.

Figure 3.5 gives the principle structure of photomultiplier tube.

 
Fig.3.5 Principle of PMT
3.9 Instrumentation System Setup

The system is based on the development of previously existing experimental system in physics department, Indiana State University [15]-[17].

The main part of the system consists of two photomultiplier tubes mounted back-to-back. One signal works as start signal the other works as stop signal. Figure 3.6 is a diagram of the main part of the instrumentation. HV stands for high voltage power supply.

Fig.3.6 PMT Detector: Main Part of System

 

 

As shown in Figure 3.7 of experimental system in next page, Photomultipliers are used to detect photons coming from biological tissues. Time coincidence method is the measurement technique. That means only when two events coming from each photomultiplier meet within the resolving time of coincidence analyzer can an emission of light be counted. Because the light emission from bioradiation is extremely low, amplifier unit is needed. Software controlled Multi-channel Analyzer is used to store and evaluate counting data and to provide complete flexibility in data acquisition. Discriminators are also needed in order to get

rid of low frequency signal.

PMT Voltage: 1950 V

Discriminator 1: 0.5

Discriminator 2: 0.5

TAC:

Time Range: 0.2m s

Multiplier: 1

Coarse Gain: 1

Fine Gain: 0.8

Bias: 0.07

Gate: Anti-coincidence

Strobe: Internal

Delay Box: 46 ns

Fig.3.7 Instrumentation System Setup