has prepared for the lab period. Then he or she proceeds to the setup of the experimental apparatus (see Fig. 3). The measurement chain is made up of: a NaI(T1) detector giving for every photon detected an analog electrical pulse, the amplitude of which is proportional to the
incident gamma-ray energy a preamplifier an amplifier shaping and amplifying the pulse arnultichannelanalyzer(M.C.A.) storingeventsversustheir energy in 1024 memory channels of 106bits each The '37Cs source emits a gamma ray of 661.7 keV energy and has an activity of 3.106 Bq and is enclosed in a cylinder of lead visible on the right of the photograph (Fig. 3). The tareet is made of a rod of aluminum 20 mm in diameter. ~ f t kthe r student has chosen the target-detector distance; he or she proceeds to the calibration. These operations are directed and controlled, step by step, by the program. As the measurements themselves are quite lengthy (about one hour) this program control avoids-lost timedue to improper choices. Then the student can start an experiment with a scattering angle of 30 degrees. During a part of the accumulation time, he can use the computer toreview then basic principles of Compton scattering and must answer questions that are propounded by another part of the program. These questions are related to simple notions such as calibration, resolution, etc., that the student will have to apply for the spectrum interpretation. When the measurement is over, the student calculates the energy of the photopeak and its area and supplies this information to the program. Additional information such as the characteristics of the source used (energy, activity), the kind of detector and the experiment duration must also be typed into the computer. All these data are necessary in order to simulate the experiment. Slmulatlon
The simulation program can then reconstruct the spectrum which appears onthe computer screen in a form similar to the real spectrum displayed on the M.C.A. screen (Fig. 4). The student can see the similarities and discrepancies between the simulated and experimental spectra. According to previous work (2),the photopeak is represented by a Gaussian function and the Compton distribution
:=740
E=548.6 Kev
N=360
is represented by a horizontal line for energies ranging from 0 to the Compton edge energy E,
ZnE', 2n
= --
1
+
(energy resulting from Compton scattering inside the detector with an angle of 180D)and by one-half the Gaussian function for enereies above E,. We must poini out that we have intentionally not taken into account the backscattering peaks because their shape and importance are strongly dependent on the detector geometry and presence of inert materials surrounding the detector: In o;r case, backscattering can be considered as a parasitic phenomenon which cannot he eliminated in the real exneriment hut can be easilv discarded in the model for simulation. 011the other hnnd, we have incorporated statistiral tlurruations in the model. The direct rumpnriwn of the experimental and simulated spectra allows students to realize that a model is onlv a representation of the studied phenomenon built for a-given burpose. A part of the phenomenon that is considered as parasitic in a given context (here backscattering) can become the ohject OF the study in another context. This is precisely the case of Compton scattering, which is considered as a parasitic phenomenon when studying the photoelectric effect. Then the student may do further simulated experiments choosing any scattering angles he likes. The simulated spectrum is obtained in less than a minute. He has onlv . to point . out spectral characteristics that are useful for interpretation of result (photopeak enerm, -.. width at half heieht, - . enerm .. of ~ o m p t o n k d g ektc.). , The most motivated students may use the program to check the effects of other parameters upon the spectral shape. I t is possible t o vary: the counting time, the targetdetector distance, and the source energy and activity. In order to simulate the use of other detectors, the characteristics of five detectors are stored as data in the program. No special instructions are given to the students a t this stage. Conclusion
The results recorded after one vear of experiments with third-year students show us that
