Page 217 - Radiochemistry and nuclear chemistry
P. 217

Detection  and Measurement  Techniques             201

               If  105 charge carrier pairs have formed in the detector and  C =  100 pF,  then AV =  0.16
               mV.  In practice the effective voltage drop A Vef f is somewhat  less than the calculated  AV.



               8.2.2.  Basic counting systems
                The block diagram of Figure 8.4 indicates the most common components  in a measuring
               circuit.  The detector (in its shield A) is connected to a stabilized bias voltage power supply
               B  which  furnishes  the  potential  difference  necessary  for  the  detector  to  operate.  The
               magnitude  and  type  of  the  signal  from  the  detector  varies,  depending  on  the  type  of
               detector,  for the type of circuit illustrated  in Figure 8.3  from a few hundred microvolts to
               several volts.  In most cases it is necessary or preferable to have a preamplifier connected
               directly  to  the  detector,  cf.  w   The preamplifier  often increases  the pulse  size to  the
               0.1  -  10 V preferred for the auxiliary system. This initial amplifier also supplies the power
               neeMed to drive the load presented by the connecting cable and input impedance of the main
               amplifier C.  The total gain in the system may vary from  10 for a Geiger-Miiller counter
               to  10 4 for some solid  state detectors.
                A limit is set on the voltage gain of an amplifier by the presence of electronic noise at the
               amplifier  input  due  to  thermal  noise  in  the  detector  and  preamplifier  input  stage,
               microphonics,  and components which cause small,  random voltage changes.  This noise is
               amplified with the signal  and can mask small pulses from the detector.  Some of the noise
               may be eliminated  by proper design and operation of the electronic  circuitry,  but a  small
               inherent noise is always present (Fig.  8.5(a) and w   The signal to noise ratio is a ratio
               of output pulse (for a given input signal) to the noise level at the output.  Noise pulses can
               be rejected by a discriminator D which  serves as a filter to allow only pulses of a certain
               minimum size to be passed on to the rest of the system;  in Figure 8.5(a)  the discriminator
               rejects all pulses below the dashed line. The signal pulse, after amplification,  is sufficiently
               large to operate the electronic  counter H  in Figure  8.4 causing  registration  of the pulses.




                    DETECTOR ARR,~IGDIENT
                                        B                 lr  r   RATE:    J
                                          B! AS SUPPLY       llE"I'ER   ,,--  Din,- ......   RECORDER
                                             l        I
                      t PREAIIPL F [ ER 1    t        I   ,
                            [
                               L        lCaAIN ^~PLlrI~   I   G MULTICHANNEL   ._~,. ..... t K   COMPUTER
                                                            ~t~YZD
                                             !                          I
                                                                       I
                                        D                 H  C~NTB-
                                         DISCRIMINATOR       TIIIB   -,-qt,--  f  ....   PRINTER
                                             1                      /   :  I

                    LEAD (OR IRON) SHIELD   E SINGLE CHANNEL   [  COUNTER-   H  SAIIPLg CHANGD
                                                             T[IID
                                           ANALYZER

                          FIG.  8.4.  The  most  common  pulse-type  measuring  units  and  combinations.
   212   213   214   215   216   217   218   219   220   221   222