(or the instructor the scores). I t is of course much safer and more convenient to score an exam from such a grid than looking for circled letters in the body of the exam. The two key programs, QUESTION FILER and EXAM PRINTER, are designed for maximum speed and user convenience; they employ several advanced and possibly novel techniques which may be of interest to those interested in programming. However, no programming experience is required to use the system with the peripherals noted, and only very rudimentary knowledge for the modifications.
Microcomputer Use in Practical and Simulated Experiments of Gamma Rays Scattering by Outer Shell Electrons D.Cabrol, G. Mallet, and T. Chauvet
Universite de Nice F-06034-Nice-Cedex, France Modeling and simulation are more and more commonly used in science and science teaching. Computer is particularly appropriate in situations in which: the experimental auparatus is not available, andlor the hazards encounteredare too high, and/or time must be expanded or shortened, etc. .\lumerous examples of compnrer simulation have heen reportrd in T I ~ I.IOURNAI.. S hlost of them are prceented in a black box format in u,hirh the model is not explicitly known hy the student. This may be eificucious for sume purposes. tur heluine but it is nor the hest nnoroach .. . the srudent obtain a clear understanding of the modeling-simulation process. Moreover. an increasine number of laboratorv instruments provide rheir results th;ough a c o m p ~ ~ t e r - l i k e ~ d (digii~~,la~ tal di.;plily, video screen, printrd output); tl~us,we think it important that students nut confuse real and simulated exveriments. In particular it must he stressed that the model is simplified representation of the system, built in order to reproduce those aspects of the system that we wish to investigate. In this sense, the model has no objective existence by itself but must be considered as a conceptual tool that may he modified in accordance with the eoals nursued. In order to have the students realize t 6 s very fnndamental nature of a model it is not sufficient for them to use simulatim programs.'rhey should be ahle to design and test models of their o w . The direct combination of real experimentation with computer simulation can be useful to ;ompare model responses with measurement. In the following article we wish to report our experience with the use of such combination by students during a threehour lab period. The spectrometric study of the Compton effect was chosen not only for its general scientific interest, but also because practical experimentation by students is made difficult by: the high cost of the apparatus (especially if one wants to use several detectors), the safety standards limiting gamma source activity, the length of measurements and their number (during which the student is inactive), and stray radiations originating from the materials surrounding the detector as well as of source origin. In such a situation, i t is common practice to use a computer-simulated experiment to overcome the difficulties. But, for the reason indicated above, insofar as possible, simulation should not replace genuine experiments. In the experiment described here, the microcomputer
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a
Program and students notes (in French) available upon request, send two blank diskettes (one to cover handling and postage from overseas). An English-language version is available from Project SERAPHIM. Details concerning modeling of detectors available upon request. 1026
Journal of Chemical Education
Figure 3. The measurement chain and lhe microcomputer. On the front: right side, the source in its lead cylinder; in the middle, the target; left side, the detector and preamplifier. On the back: right side, lhe minocomputer displayins the simulated spectra: in the middle, theamplifier:isn side, the multichannel
displaying me experimentai
(Apple I1 48k)2 is used to: give the student a short review on the principle of Compton scattering; guide himher in the setting up of experimental apparatus; check the calibration and control the student's choices; test hisher understanding of calibration procedure; produce a simulated spectrum of the real experiment conducted by the student; and extend the field of investigation through interactive simulation. Modellno " Corn~tonEffect
When an X- or gamma beam of u frequency photons strikes a target containingfree electrons, we can observe, in a direction making a 0 angle from the incident direction, a scattered beam of u' frequency photons. This effect, called "Compton scattering," has had a great theoretical importance, since it contributed, with photoelectric effect, to justify the radiation corpuscular features: incident photon behaves as a particle of initial energy E, = hu giving up a part, E,., of its energy to the target electron. Moreover, together with the photoelectric and pair effects it constitutes one of the three principal modes of interaction of gamma-rays with matter. This effect has numerous practical applications in various fields such as: industry, radiotherapy, earth science, etc. The modeling of the Compton effect is developed by students (chemistry and physics students a t third-year level) with the help of a teacher during prelah sessions. On the one hand, the laws of conservation energy and momentum for the interaction allow the calculation of the energy (E', = hv' of the scattered gamma-ray) as a function of the incident energy E, and of the scattering angle 8. On the other hand, the Klein-Nishina formula (1) gives the cross section a, of the Compton interaction of unpolarized photons with unoriented electrons as a function of the incident y-ray energy and scattering angle 0. Moreover, during a previous laboratory experiment, the student has recorded the response of a NaI(T1) detector versus the energy of incident gamma-rays (I', energy resolution; r, detector efficiency; and r, ratio of the pulse height to the Compton distribution). T h e modeling of the detector response has been carried out by one group of students, and the resulting model is explained to the other students. Parameterisat& of the model for other detectors is included in the modeL3 Experiment
First, the student must answer some simple questions that are asked by the computer in order t o verify that he or she
