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Laboratory Experiment 1_ Introduction to Counting Radioactive

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					Laboratory Experiment 1: Introduction to Counting Radioactive

 Samples: Gas Ionization Detectors – Geiger Muller Detectors




                        Michaela Flak
                        Experiment Performed: 6-16-04 / 6-18-04
                        Due Date: 6-23-04
                        Partner(s): 6-16-04 Chris Seyvert and Nick Smith
                                    6-18-04 Kiel Holliday
                        Grader:
Abstract

        The purpose of this laboratory was to become familiar with the operation and

theory of Geiger Muller counters. In order to become acquainted with the GM detector,

experiments examining the characteristics of the detector were performed.

Characteristics examined included: detector sensitivity, detector response, detector

efficiency, detector resolving time, and counting statistics. As a result of these

investigations, it was found that the GM is a very useful detector for many reasons. This

instrument is very easy to use and relatively inexpensive while being a very effective

radiation detector.



Introduction

        The purpose of this experiment was to become familiar with the operation of

Geiger Muller (GM) detectors. The GM detector is a tube filled with argon gas which

has a thin window allowing radiation to enter. The incident particle and the mass of the

detector exchange energy. This energy exchange is measured in order to detect the

radiation. The GM detector consists of an anode and a cathode. The anode is a

conducting wire that is suspended within the tube. The cathode is the outer shell of the

tube. The application of a voltage to the GM detector causes the radiation and the

interior gas to interact (1).

        If the incident particle is an alpha or beta particle, the electric field of the particle

will interact with the argon gas. The result is that the particle decelerates while the

orbital electrons of the gas within the GM detector accelerate. The energy transfer must

me sufficient enough to ionize the gas in order for an electron-ion pair to be created. The
average number of electron-ion pairs created is proportional to the energy that is

deposited in the gas. The application of voltage causes the ions to accelerate to the

cathode and the electrons to accelerate towards the anode. The gathering of the electrons

at the anode is registered as a pulse by the detector. Eventually the ionized gas will

collide with the cathode causing the release of additional electrons that are not factored

into the proportionality between the number of pairs and the energy. In order to prevent

this from affecting the data, a fill gas is added. The molecules in the fill gas absorb

energy from the ionized argon atoms causing the energy of the ions to reduce to below

the ionization potential of the cathode. This prevents emission of electrons by the

collision of the gas with the cathode such that these collisions will not effect the data

collected (1).

       The Geiger Muller Detector does not distinguish between the different types of

radiation. Geiger Muller Detectors are operated in the 750 – 1000 V range. The detector

operates in this range in order to prevent damage to the instrument. At voltages above

1000 V, the tube would continuously discharge resulting in damage to the detector. At

voltages below 750 V, the detector gas will not discharge enough to produce the same

number of electrons as the incident radiation. Operating the detector within the 750-1000

V range allows each incident particle to produce a pulse in the detector (1).

       Alpha particles have an increased response to current compared to beta particles

in the detectors operating range because ionization, I, produced by a particle is related to

the particle’s mass, m, energy, KE, and the charge, Z. Due to the significantly larger

mass and charge of an alpha particle compared to a beta particle, alpha particles create

much more ionization per particle and thus an increase in current.
                                                     mZ 2
                                                  I                                        1
                                                     KE



       The characteristic voltage response curve is determined for each individual Geiger

Muller detector in order to find the optimum operating voltage for that individual

instrument. The slope of the plateau region of the curve is found using the following

equation:



                                                ( R2  R1 )
                                       100 *
                                                    R1
            % Slope / 100Volts                             *100                                2
                                               V2  V1


       All counts from the detector must be corrected for background and dead time.

Dead time (or resolving time) is the amount of time that it takes for the detector to

recover from the collection of a count. Resolving time is calculated with the following

formula:



                                               R1  R2  RT
                                        Tr                                             3
                                                  2 R1 R2

       This formula is used by finding the counts with half sources. R1 and R2 represent

the counts found with the active source on the right and left side respectively. RT is the

counts found when both halves of the source are active.
       The true count rate, R, can then be found for the observed count rate, Ro, using the

following formula:

                                                 Ro
                                         R                                         4
                                              1  Ro Tr



       The efficiency of the detector can be found utilizing:



                                          Am (dps)
             %efficiency                                        x100           5
                             As (mCi ) * 2.22 x10 6 (dps / mCi )



                                              Or

                             A
                              100                                         6
                             AA


       Where,

                                           R(cpm)
                      Am (dps)                                              7
                                   (60 sec/ min) * %decay




Apparatus and Experimental Procedure

       Due to the availability of instrumentation, the detector used throughout this

experiment varied. A Spectech ST-350 Counter was used to determine the characteristic

voltage curve of the detector. A Model 575 Scalar-Parameter detector was then used to

determine the relationship between the source distance from the detector and the
efficiency of the detector. Three samples of no-carrier added 59Fe were prepared. A

Spectech ST-350 was then used to explore the effects of backscattering with several

different backing materials. This same instrument was used for the remainder of the

experiment. The consistency of the samples prepared previously was then examined by

counting each of the three samples. Next, a sample of 59Fe was counted 50 times for use

in statistical analysis. Background readings were then collected. Finally, the dead time

of the instrument was found. A detailed explanation of the experimental procedure can

be found in reference (1).



Raw Data

       Three background counts were taken:

               18CPM            25 CPM        32CPM

       Thus, the average background used for all background corrections was:

               25 CPM.



Appendix A contains all tables and charts:

       Table 1: Contains the resolving time data.

       Table 2: Contains the data pertinent to the determination of the characteristic

               voltage curve. Source used: 204Tl 1.0 µCi s/n 51E18.

       Table 3: Contains the data relevant to the counting rate vs. distance. Data

               received from Kiel Holliday. Source used: 204Tl 1.0 µCi s/n 51E18.

       Table 4: Contains the data collected for the backscattering of 59Fe. Source used:
               204
                     Tl 1.0 µCi s/n 51E18.
       Table 5: Contains the reference data for the exploration of detector response to

              different types of radiation.

       Table 6: Contains the experimental data for the exploration of detector response

              to different types of radiation.

       Table 7: Contains the counts found for the determination of consistency in the

              three 59Fe samples.

       Table 8: Contains the counting statistics for the 50 counts of 59Fe. Data received

              from Jill Pinter and Greg Lillvis.

       Table 9: Contains the background analysis. Data received from Laura Altman

              and Dustin Demoin.



Appendix B contains all graphs and figures:

       Figure 1: Graph of the characteristic voltage curve. Source used: 204Tl

              1.0 µCi s/n 51E18.

       Figure 2: Plot of the slope of the characteristic voltage curve plateau.

       Figure 3: Plot of the count rate vs. the inverse square of the distance from

               the detector. Data received from Kiel Holliday. Source used: 204Tl 1.0

               µCi s/n 51E18.

       Figure 4: Graph of the backscattering. Source used: 204Tl 1.0 µCi s/n

              51E18.

       Figure 5: Graph of efficiency vs. maximum beta energy.

       Figure 6: The Gaussian distribution of the 50 data points collected in the

              statistical analysis. Data received from Jill Pinter and Greg Lillvis.
        Figure 7: Graph of the background data. Data received from Laura Altman and

                Dustin Demoin.



Data Analysis

        As presented in the raw data, a background collection was necessary in order to

accurately report the counts collected. This background was subtracted from all data

reported as “background corrected.” The average background collected for this

experiment was



                25 CPM.



        In addition to the background correction, a resolving time correction was also

necessary. The resolving time is the amount of time that it takes for the computer to

“catch up” with the pulses entering the detector. This data was found by using two half

sources and a blank the size of a half source. See Table 1 for data related to the resolving

time. All counts greater than 5000 CPM are corrected for resolving time. The resolving

time is calculated using Equation 3:



       44517  44381  82583
TR =                                     1.60 x 10-6 µmin/count = 96 µsec/count
          2(44517)(44381)


        The actual count rates are found using equation 4:
                             82583
               R=                               = 95156 CPM
                    1  82583(1.6 x10 6 )


       The operating voltage of the GM detector is found by plotting the counts vs.

voltage as in Figure 1 or the data in Table 2. By analyzing Figure 1, a plateau region can

be found. This plateau region represents the operating voltage of the detector. The

plateau region for this detector ranges from



               950-1000 V.



       For the purpose of this experiment, the operating voltage was selected to be 975

V, as shown on Figure 1. This operating voltage was chosen because it was the mid-

range of the plateau region. Figure 2 is representative of the slope of the plateau region.

Utilizing the data found in Table 2, the % Slope/100V can be found using Equation 2:




                                       (1.296 x104  1.301x104 )
                                 100
               % Slope/100V =
                                                 1.301x104               100   = -.828
                                               1000  950


       By analyzing Table 3 and Figure 3 it can be seen that there exists a linear

relationship between the count rate and the inverse square of the distance between the

source and the detector. The equation of the line that was found was:
                       Y = 947912x + 3441.4



       Table 6 contains the data relevant to the efficiency of the GM detector. Figure 5

represents the efficiency vs. the maximum energy of the beta particle that is emitted.

This data shows that a particle with more energy can be more easily detected.      The

uncertainty in the reference sources was assumed to be 20%. The efficiency is calculated

using Equation 5:



                       33
                         P:    (6/3663)100 = .16 %



       Table 7 shows the counts collected from the samples of 59Fe that were prepared.

Two of the samples have very close count rates, whereas one was slightly further off

(though still reasonably close). This variation may be accounted for in that one person

pipetted two of the samples and the other person pipetted the final sample. These

samples were not labeled to distinguish who pipetted them so it is not certain if this is an

accurate explanation of the deviation in count rates.

       Table 8 contains the data for the 50 counts of 59Fe. This data was plotted and

fitted to a Gaussian curve in Figure 6. Aside from the last data point, the data fits the

Gaussian very well.

       Table 8 and Figure 7 represent the background analysis. This data shows the

relationship between the uncertainty and the duration of measurement.

       Table 4 and Figure 4 demonstrate the linear relationship between the count rate

and the atomic number of the backing material. This is representative of backscattering.
Discussion:

       The characteristic voltage curve plateau had a slope of -.878 %/100V. This slope

should not be negative for a Geiger Muller detector. However, the error in the data points

is significant enough that it could make the slope positive. It can be clearly seen from the

error bars in Figure 2 that the slope could have been positive.

       The efficiencies that were found for the Geiger Muller detector demonstrate that

this detector was not sufficient for detection of weaker beta and gamma emissions. A

more sensitive detector should be used for measurement of such sources. The Geiger

Muller detector is an effective and cheap method of detecting energy emissions aside

from such weak beta and gammas.

       The background data found in Table 9 demonstrates that longer time spans of data

collection are ideal for detecting radiation. As the count time increases, the error bars on

the corresponding Figure 7 decrease. A slight increase in the duration of data collection

decreases the error significantly.

       The data for the Gaussian Curve, Figure 6, was as expected. The last data point

did vary from the Gaussian, but overall the data fit the Gaussian model very well.

       The relationship between count rate and sample-detector distance is shown in

Figure 3. The data was plotted as the inverse square of the sample-detector distance. A

linear regression fit the data points with a regression coefficient of



               R = .9867.
This regression coefficient is very near 1, signifying that the data falls nearly in a straight

line. A linear relationship would be expected with the geometric inverse square law

effect.



Conclusion

          Geiger Muller detectors are very easy instruments to learn how to use. This

detector provides accurate and meaningful data without the user requiring too much

training. The methods and theory learned in this experiment will be beneficial

throughout this course as various detectors are utilized. The Geiger Muller detector is an

efficient radiation detector while much less costly than some of the other detectors that

will be used in this course.
                                     Appendix A

Table 1:
                              Resolving Time Adjustment

  Shelf    Split Source Location   Observed Count Rate (CPM)    Resolving Time (CPM)
   3                right                    44517                       1.6E-06
   3               whole                     82583
   3                 left                    44381
   4                right                    28198                        1.5E-06
   4               whole                     54344
   4                 left                    28500



Table 2:
                    Determining the Characteristic Voltage Curve

                                                            Counts
                                   Counts Corrected       Corrected for
 Voltage   Counts     Sigma         for Background         Dead time       CPM
   (V)     (CPM)      (CPM)              (CPM)               (CPM)        Correct
   100        0         0                    0                     0         0
   200        0         0                    0                     0         0
   300        0         0                    0                     0         0
   400        0         0                    0                     0         0
   500        0         0                    0                     0         0
   600      1668       40.8                1643                  1668      1643
   700      4350       66.0                4325                  4350      4325
   800      5224       72.3                5199                  5224      5199
   850      5027       70.9                5002                  5027      5002
   875      5350       73.1                5325                  5350      5325
   900      5481       74.0                5456                  5481      5456
   925      5606       74.9                5581                  5606      5581
   950      5891       76.8                5866                  5891      5866
   975      5886       76.7                5861                  5886      5841
  1000      5869       76.6                5844                  5869      5844
  1025      5965       77.2                5940                  5965      5940
  1050      6111       78.2                6086                  6111      6086
  1075      6256       79.1                6231                  6256      6231
  1100      6070       77.9                6045                  6070      6045
Table 3:
                                                                       59
                        Counting Rate vs. Distance at 600V with         Fe

              Shelf distance        counts                              Uncertainty     Net     True Count
 Shelf #          (mm)                (#)      Time(sec)        CPM       (CPM)       Counts    Rate (CPM)
   1               5.2               10239         17          36137       190.1      36131        38231
   2              14.7               10341         60          10341       101.7      10335        10500
   3              24.7               10305        140           4416       66.5        4410         4440
   4              34.1               10341        258           2404       49.0        2398         2407



Table 4:

                                   Backscattering Using 59Fe


               Atomic # of             CPM Background            CPM Dead        Uncertainty
  Absorber      Absorber       CPM        adjusted             Time Corrected      (CPM)       Adjusted CPM
    None             0         13077       13052                   13356            114.4          13331
   Plastic*         6*         14910       14885                   15274            122.1          15249
  Aluminum          13         15175       15150                   15552            123.2          15527
    Lead            82         19675       19650                   20314            140.3          20289
 *carbon used for plastic



Table 5:

           Detector Response to Different Types of Radiation Reference Data

                                    Reference
Source               Fractional     Activity       Reference
Type       Source    Abundance      (uCi)          Date
beta       204Tl           0.971             0.6    5/1/2002
beta       33P             1.000             0.1   6/18/2004
beta       32P             1.000           0.12    6/18/2004
beta       14C             1.000              10    1/1/2001
gamma      57Co            0.856               1   6/18/2004
Table 6:

       Detector Response to Different Types of Radiation Experimental Data
Counts taken: 6-18-04 5:00pm

                                   CPM
                                   Background                            Decay
                                   and Dead        Count                 Corrected     Decay
           Elapsed                 Time            Time       Activity   Activity      Corrected        %             Uncertainty
Source     Time          Counts    Corrected       (sec)      (dps)      (uCi)         Activity (dps)   Efficiency    (CPS)
204Tl      778 days        4063            13425      18.48      224            0.41           15170           1.52         3.45
33P        6 hrs.          4007              351     600.62          6        0.099             3663           0.16         0.11
32P        6 hrs.          4030             6069       39.9      101            0.12            4440           2.28         1.59
14C        1264 days       4242            12190       21.2      203              10         370000            0.05         3.07
57Co       6 hrs.           977              159     280.68          3             1           37000           0.01         0.11



Table 7:

                                         Sample Consistency
Counts taken: 6/18/04 5:30pm

 Sample        CPM        Background Corrected           Sigma
   1           1521             1496                       38.7
   2           1086             1061                       32.6
   3           1612             1587                       39.8



Table 8:

                                          Counting Statistics

Raw Data                                                                 Average            147.8
      120          137            146       151        158               STDEV          12.458912
      124          138            147       151        159
      126          139            147       152        160               Bins          Freq.     Gaussian
      130          140            148       154        161                       110            0 0.0603425
      133          140            148       154        161                       120            1 0.5811299
      133          142            148       155        163                       130            3 3.1858193
      134          144            148       155        164                       140           11 9.4546297
      134          145            149       157        165                       150           15 15.222129
      134          146            150       157        167                       160           13 13.309152
      136          146            150       158        186                       170            6 6.3174866
                                                                                 180            0 1.6255056
                                                                                 190            1 0.0176594
                                                                                               50 49.773854
Table 9:

                                    Background Analysis

              Counting   Total      Total      Total      Total      Total
Sample        Time (s)   Counts 1   Counts 2   Counts 3   Counts 4   Counts 5   Average     CPM   STDEV
Background          6           3          3          1          2          2          2     20       10
Source: Cd-
109                 6          88         94         93         88          89         90   900       90
                                                                     Net Counting Rate      900       90
                                                                     Relative Percent
                                                                     Error                          20%

Background         30          13         17         14          9         13         10     20        6
Source: Cd-
109                30        427        452        473        461          427        448   896      42.3
                                                                     Net Counting Rate      880        40
                                                                     Relative Percent
                                                                     Error                            9%

Background         60          22         23         23         27         39         27     27       5.2
Source: Cd-
109                60        880        877        896        852          901        881   881      29.7
                                                                     Net Counting Rate      854        30
                                                                     Relative Percent
                                                                     Error                          7.0%

Background        300        134        117        139        145         131        133     27      2.31
Source: Cd-
109               300       4612       4526       4482       4563        4551        4547   909     13.49
                                                                     Net Counting Rate      882      13.7
                                                                     Relative Percent
                                                                     Error                         3.11%
                                            Appendix B


Figure 1:

                                    Characteristic Voltage Curve




Figure 2:


                    Slope of Characteristic Voltage Curve Plateau    y = -0.44x + 6311
                                                                        R2 = 0.9098
                   6100
                   6050
                   6000
   Counts/30 ses




                   5950
                   5900
                   5850
                   5800
                   5750
                   5700
                   5650
                       940    950    960      970     980      990    1000      1010
                                                Voltage
Figure 3:

                       Count Rate vs. the Inverse Square of the
                              Distance from Detector

                      50000
   Count Rate (CPM)




                      40000       y = 947912x + 3441.4
                                       R2 = 0.9867
                      30000

                      20000

                      10000
                         0
                              0    0.01         0.02         0.03       0.04
                                               1/d^2




Figure 4:




                                          Backscattering

                      25000
   Count Rate (CPM)




                      20000

                      15000
                                                       y = 75.602x + 14190
                      10000                                R2 = 0.9528

                      5000

                         0
                              0    20          40           60         80      100
                                          Atomic Number of Backing
Figure 5:

                                                                  Efficiency vs. Max Beta Energy

                                                                                 Efficiency vs. Max. Beta Energy



                           5.00E+00


                           4.50E+00


                           4.00E+00


                           3.50E+00


                           3.00E+00
                                                                                                                                           y = 0.0025x - 0.0791
              Efficiency




                           2.50E+00
                                                                                                                                               R2 = 0.9532

                           2.00E+00


                           1.50E+00


                           1.00E+00


                           5.00E-01


                           0.00E+00
                                 0.00E+00   2.00E+02   4.00E+02    6.00E+02     8.00E+02     1.00E+03     1.20E+03   1.40E+03   1.60E+03   1.80E+03
                                                                                    Energy (keV)




Figure 6:
                                                                                Gaussian Curve
                                                                              Normal Distribution of Counts                                    Gaussian
                                                                                                                                               Count Data
                 16



                 14



                 12



                 10
  Frequency




                       8



                       6



                       4



                       2



                       0
                                 110           120          130           140              150             160         170          180           190
                                                                                   Number of counts
Figure 7:


                    Background Analysis

         1000
          980
          960
          940
          920
   CPM




          900
          880
          860
          840
          820
          800
                1       10                100   1000
                             Time (min)
                                    Appendix C

References:


1. CHE-362 Laboratory Manual. Summer School in Nuclear and Radiochemistry June

       14 – July 23, 2004. Brookhaven National Laboratory.

				
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