Syllabus

Goals

The purposes of Junior Lab are to give you hands-on experience with some of the experimental basis of modern physics and, in the process, to deepen your understanding of the relations between experiment and theory, mostly in atomic and nuclear physics. You will do experiments on phenomena whose discoveries led to major advances in physics. The data you obtain will have the inevitable systematic and random errors that obscure the relations between the macroscopic observables of our sensory experience and the ideal laws that govern the submicroscopic world of atoms and nuclei. You will be challenged to learn how each of the experimental setups works, to master its manipulation so that you obtain the best possible data, and then to interpret the data in light of theory and a quantitative assessment of the errors. We think you find satisfaction in observing, measuring and understanding phenomena many of which would have won you the Nobel Prize if you had discovered them.

Organization - Fall Term Breakdown

The first two sessions familiarize you with the lab and give everyone a common foundation in experimental techniques, data analysis and computing tools including MATLAB® and LaTeX. Two 3-hour sessions are scheduled for two of four short introductory experiments: Optics, Poisson Statistics, Transmission of Electromagnetic Pulses and The Photoelectric Effect. To learn pulse propagation techniques, one of your two introductory experiments must be either Poisson Statistics or Electromagnetic Pulses. The preparatory questions and one written summary resulting from these preliminary experiments will be graded (to let you know how you're doing) but not recorded, so your final grade will be unaffected. Following this introductory period, students will plan and execute four longer experiments in four 3-hour sessions. The term culminates in a week-long series of public oral presentations given by students to peers, friends and faculty in an American Physical Society conference type event.

Ethics in Science and Science Education

When you read the report of a physics experiment in a reputable journal you can generally assume it represents an honest effort by the authors to describe exactly what they observed. You may doubt the interpretation or the theory they create to explain the results. But at least you trust that if you repeat the manipulations as described, you will get essentially the same experimental results.

Nature is the ultimate enforcer of truth in science. If subsequent work proves a published measurement is wrong by substantially more than the estimated error limits, a reputation shrinks. If fraud is discovered, a career may be ruined. So most professional scientists are very careful about the records they maintain and the results they publish.

Junior Lab is designed to provide preprofessional training in the art and science of experimental physics. What you record in your lab book and report in your written and oral presentations must be exactly what you have observed including date, time and who did it.

Sometimes you'll get things wrong because of an error in manipulation, equipment malfunction, misunderstanding, or a miscalculation. The instructor's job is to help you figure out what went wrong so you can do better next time. If circumstances in an experiment are such that you cannot get your own data (e.g. broken equipment, bad weather), you may use somebody else's provided you acknowledge it.

Fabrication or falsification of data, using the results of another person's work without acknowledgement, and copying from "the files" are intellectual crimes as serious as plagiarism, and possible causes for dismissal from the Institute.

The precaution about the acknowledgement of other people's data also applies to acknowledging other people's rhetoric. The appropriate way to incorporate an idea which you have learned from a textbook or other reference is to study the point until you understand it and then put the text aside and state the idea in your own words.

One often sees, in a scientific journal, phrases such as "Following Albert Einstein ..." This means that the author is following the ideas or logic of Einstein not his exact words. If you quote material, it is not sufficient just to include it in the list of references at the end of your paper. You should use the following formatting:

The quote should be indented on both sides or enclosed in quotes, and attribution must be given immediately in the form of a reference note.

Importing text from a published work, from other student papers, or from the labguide without proper attribution is a serious breach of ethics and will be dealt with by the Committee on Discipline.

Most Junior Lab experiments are concerned with data comparison measurements of well known fundamental constants such as h, e, k, e/m, G, or significant physical quantities such as the mean life of the muon or the cross section of an electron for scattering a photon. The purpose of these experiments is to give you hands-on experience with atomic and nuclear phenomena, a sense of the reality of the concepts and theories you have studied in books and lectures, and the beginning of professional skill in obtaining and recognizing reliable data and extracting meaningful results from them. There is nothing wrong with "peeking" in the CRC Handbook or any of the many relevant texts to see what your experiment should have yielded. Indeed, the way to get maximum benefit from your Junior Lab experience is to play it as a game in which you squeeze the most accurate measurement you can get out of the available equipment and the practical limits of analysis, make a rigorous estimate of the error, and then compare the results with the established value. If the established value is outside your error range, try to find out what went wrong, fix it, and try again. If the established value is in your error range, don't rest easy, but do whatever may be necessary to prove it isn't an accident. Repetition is the essential key to attaining confidence and errors for a result, whether of a single measurement or an entire experiment! But whatever the outcome of an experiment is, you must tell exactly what you observed or measured when you present your oral or written report, regardless of how "bad" the results may appear to be.

Required and Recommended Text

Required Texts

1. The Junior Lab Reader.
   
   
2. Bevington, Philip R., and D. Keith Robinson. Data Reduction and Error Analysis for the Physical Sciences. 3rd ed. Boston, MA: McGraw-Hill, c2003. ISBN: 0072472278.

The Bevington and Robinson text contains a comprehensive treatment of error analysis and will be useful throughout your career.

Other Texts to Regularly Consult

There are several other recommended textbooks:

• Melissinos, Adrian. Experiments in Modern Physics. 1st and 2nd ed. Burlington, MA: Academic Press, 1966 and 2003.
  Please consult these before and during your investigations. Material essential to the understanding of an experiment which can be found in the Melissinos text is generally omitted from the Lab Guide. Note that the Physics Reading Room has both editions which offer different material, you should consult them both!
  
  
• Preston, Daryl, and Eric Dietz. The Art of Experimental Physics. New York, NY: John Wiley & Sons, 1991.
  
  
• Taylor, John. An Introduction to Error Analysis. 2nd ed. New York, NY: University Science Books, 1997.
  This book covers much of the same material as Bevington and Robinson.

Grading Policy

Attendance and Lab Performance

The regularity of your attendance will be a factor in determining your grade in the course. Also your preparedness for the measurements and alternating as the "lead" (with your partner), to carry them out. Lab instructors will be delighted to accommodate your improvements and corrections to the lab guides! The lab will be closed at 5pm each afternoon.

It is essential that you efficiently use all of the laboratory time assigned to you, and sometimes more. An experienced experimental physicist will be present in every scheduled session. He or she will be assisted by a graduate teaching assistant. In addition, the Junior Lab staff includes two technical instructors responsible for the maintenance of the equipment and the development of new experiments. We are ready and eager to help you make things work properly and answer questions. Call for help when you get stuck.

Laboratory Notebooks

One critical objective of this course is to instill habits of record keeping that will serve you well in future research. To this end you will be given a standard experimental notebook in which the complete dated record of procedures, events, original data, calculations and results of every experiment is to be kept. No other form of notebook is acceptable in this course. Though you will generally work in pairs and are urged to collaborate in all aspects of carrying out the experiments and analyses, each student must keep a complete, dated record of each experiment and its analysis. The cross-hatched paper in the Computation Book is convenient for formatting tabulations, and for guiding line drawings and making rough plots. High resolution plots, photos, and Xerox copies of shared data should be glued or taped in place. You must write a sufficient narrative as the experiment proceeds so that, years later, you could reproduce the results you obtained. Notes, tables, and graphs should be neat and compact, leaving as little empty space in the lab notebook as is compatible with clarity and the logic of organization. There should be no loose sheets or graphs floating around.

Analyze data in the lab in a preliminary way as you go along to check for reasonableness. If you are making a series of measurements of one quantity as you vary another, plot the results as you go along so that you can see the trend, catch blunders, and judge where you may need more or less data. Repeat every measurement at least three times in as independent a manner as possible in order to establish a statistical basis for estimating random error and to reduce the chance of blunders. If you get through all the manipulations and preliminary analysis of an experiment in less than the four regularly assigned lab sessions, take the opportunity to perfect part or all of the experiment so as to obtain the best possible data set.

Many experiments will require you to transfer your data to a computer and store them in files on disk. Obviously, it is not practical in these cases to print out all your data and paste them into your notebook. However, we expect to see, in your lab notebook, representative plots or tables. In addition, we expect a clear description and summary of the data files so that when you return to the data days or weeks later, you are able to identify particular files with procedures you carried out in the lab.

Student notebooks will be evaluated three times during the Fall term. The first will follow the Introductory experiments and be 'graded but not recorded' in order to help students learn what is expected. There will then be two recorded notebook evaluations, following the first and third main experiments by the Instructor to be conducted during lab sessions. Please talk with him before your notebook evaluation if you have any questions that are not answered in the reader.

Preparatory Questions and Data Analysis Assignments

Each lab guide has a set of preparatory questions which point you to the essentials of the experiment. Before the first session of each experiment you are expected to work out the solutions to the preparatory problems and/or predictions in your lab book. Make a Xerox copy of your solutions and deposit it in your TA's mail box. It will be collected shortly after the start of the first session. Late solutions will not be accepted because you will need to know this material before the experiment. Your solutions will be graded by the graduate teaching assistant and returned at the next session. The Introductory Experiment preparatory questions will be graded for feedback but will not count towards your final course grade.

During the first two weeks of lab sessions, in conjunction with the short introductory experiments, you will be required to complete two written assignments on data reduction and error analysis. A result like

We find c = 2.5 × 10¹⁰cm s^-1 is meaningless without an error according to C.F. Gauss!

c = (2.5 ± 0.5) × 10¹⁰cm s^-1 is acceptable.

c = (2.5 ± 0.05) × 10¹⁰cm s^-1 is plain wrong!

These assignments are designed to teach you some of the fundamentals of data analysis, including parameter estimation and modeling of data using maximum likelihood techniques. You will be expected to use these techniques in the analysis of all your Junior Lab experiments.

Oral Examinations (4 Private)

A one hour total length (2 students x 30 minutes each) oral examination and discussion of each main experiment will be held between the pair of students and one or more of their instructors within 10 days of the last scheduled session for that experiment. To familiarize you with the procedure, a one-hour oral will be held on one of the two introductory experiments of the students choice. This oral will proceed identically as the others and will be 'scored' but will not count towards the students final course grade. It is designed to give the student feedback on content, style and presentation without the pressure of a graded performance. Partners should choose different introductory experiments for this initial oral exam. Videos of these practice orals will be used along with guidance and advice from Lecturers from MIT's Program in Writing. Students should schedule a 1-hour appointment with one of these staff within the week following their practice oral for feedback.

Each student must bring to the exam session his or her lab notebook. Each student should prepare a 15-minute oral report on the theoretical and experimental aspects of a single portion of the experiment. Fifteen minutes is a short time, so it is essential that you rehearse your presentation as you would if you were giving a 15-minute paper at a meeting of the American Physical Society. Please review the Society guidelines. We suggest a maximum of ten slides and strongly suggest preparing your presentation electronically (e.g. LaTeX or Microsoft® Power Point®) and using the LCD projector for the cleanest most professional presentation possible. The Junior Lab study materials section has detailed instructions and a template for generating your own presentations. The theoretical section should demonstrate a mastery of some portion of theory relevant to understanding the significance of the experimental results. The experimental section should dominate and demonstrate an understanding of how the equipment works, what was measured, how the data were reduced, and how the random and systematic errors were estimated. Each student must discuss motivating theory and experiment; it is not acceptable to discuss theory only or experiment only. Full cooperation with lab partners and others in preparing for the oral reports is encouraged.

Final Public Oral Presentation

At the end of the term in December, each student will give a public oral presentation which will be attended by all students in the section; see the details about Public Orals below. The Institute has introduced a new "communication" requirement that includes oral presentation of material. In the Physics department, we are requiring that each Junior Lab student give a 15-minute oral presentation to the entire section on one of the experiments. This is in addition to the four jointly prepared oral presentations given to the section instructor. We have reserved the last week of class for this purpose. The public oral presentations should be given in the style of a paper presented at a conference, with careful attention paid to the preparation of material, usually in the form of an electronic presentation or transparencies, and to the clarity of the oral discussion. Questions from classmates and the audience are encouraged allowing for a general discussion of the experiment. Each student is required to make a 1-hour appointment at least four days prior to their public presentation where they should do a "dry run" and receive feedback. The dry run will not be graded, but failure to do it will result in a reduced grade for the oral presentation.

4-page Written Summaries

You must email an Adobe® PDF copy of your, individually-prepared, written summary (≤4 pages including figures) of the purpose, theory, and results of the experiment by midnight on the day you give your oral examination. The delay between oral exam and paper submission allows you to correct any egregious mistakes that were uncovered during the exam so as not to repeat them in your written work and receive a double penalty! All your work on the experiment should be summarized, not just the part you chose for your oral presentation. The individual summary you hand in should show evidence of your own mastery of the entire experiment, and possess a neat appearance with concise and correct English. Its organization and style should resemble that of a typical abstract that follows the title of an article in the Physical Review Letters. The abstract is essential. It should briefly mention the motivation (purpose), the method (how measured) and most important, the quantitative result with uncertainties. Based on those, a conclusion may be drawn. The report must be type-written in a form that would be suitable for submission as a manuscript and to aid you in this process we have produced a sample paper template written in LaTeX that we encourage you to study and use for your own submissions. A copy of this paper is also included as part of The Junior Lab Reader.

The body of the summary should include a discussion of the theoretical issues addressed by the experiment, a description of the apparatus and procedures used, a presentation of the results (including errors!), a discussion of these results, and, finally, a section briefly presenting your conclusions. The total length (including figures) of your summaries should not exceed four pages in length. It is easiest to read if you include figures and plots inline within the text and the sample paper template shows you how this is easily done. However, do not inundate the reader with material; you should find a way to summarize your results in at most two or three plots or tables. The figures and tables must be properly captioned. Material and ideas drawn from the work of others must be properly cited, and a list of references should be attached to the summary.

Papers will be graded using the following criteria:

1. Motivation - 15%

2. Experiment - 40%

3. Analysis - 30%

4. Style and English - 15%

Safety

We are fortunate that there has never been a serious injury in Junior Lab. Prevention of injury is a matter of being aware of and having respect for pieces of equipment that are potentially dangerous. Nevertheless, since it is virtually impossible to set up a reasonably comprehensive and interesting set of experiments in modern physics without using equipment which has potential hazards, it is essential that staff and students be aware of the hazards, and exercise appropriate cautions.

Electrical Safety

The first rule is never to work alone. Some years ago a student was electrocuted in Building 4 by a laboratory power supply. Had he not been by himself, someone might have saved him.

All high voltage supplies are clearly marked as dangerous. Do not poke or probe into them. Turn off the supply if you need to change cable connections. The supply may be dangerous even when turned off if the capacitors have not discharged; always keep one hand in your pocket when testing any circuit in which there may be high voltages present so that if you get a shock, it will not be across your chest. Never go barefoot in the lab. Remember that it is current that kills. A good (e.g. sweaty) connection of 6 volts across your body can kill as well as a poor connection of 600 or 6000 volts.

Laser Safety

A laser beam may not seem very bright, but if it enters your eye it will be focused by your eye lens to a pinpoint spot on the retina where the intensity is sufficient to destroy retinal cells. It is wise to terminate a laser beam with a diffuse reflector so that the beam doesn't shine around the room. Never examine the performance of an optical system with a laser by viewing the beam directly with your eye.

Cryogenic Safety

When the cap on a liquid-helium Dewar is left off air flows in and freezes in the neck, forming a strong cement. When a probe is inserted, it may be frozen in solid. Then pressure will build up until something explodes. During the superconductivity experiment, never leave the Dewar cap off for more than a few seconds. Always ream out the Dewar before you use it. Check periodically to see that the probe is free. If the probe should freeze in the Dewar, get help immediately from any of the Junior lab staff or a professor or TA.

Liquid nitrogen is chemically inert, but it can cause severe frostbite. Wear gloves and protective glasses when transferring or transporting liquid nitrogen.

Radiation Safety

Radiation safety at MIT is under the authority of the Radiation Protection Office. Junior Lab is accountable to that office for the safe handling and accountability of the sources used in the experiments. During the first class session, a member of the Radiation Protection Office will instruct you in the safe use and handling of radioactive material.

Meticulous care must be taken by all students and staff to insure that every source signed out from the repository be returned immediately after its use and signed in.

Ionizing radiation damages tissue; any exposure should therefore be minimized. The unit of radiation exposure is the rem (roentgen equivalent man). A new unit, called the Sievert (= 100 rem) is recommended by the International Commission on Radiation Units and Measurements (ICRU). Your inescapable dosage from cosmic rays and other background sources is 360 mrem yr^-1, which works out to 4.2 x 10^-2 mrem hr^-1. The recommended limit to exposure for a member of the general public is 100 mrem yr^-1, averaged over any consecutive five years. If you follow the Junior Lab guidelines, your exposure will be only a small fraction of the dose you receive from the natural background. A meter is available for you to check the radiation levels yourself.

Radioactive sources emit three types of radiation: high energy helium nuclei (alpha rays), electrons (beta rays), or photons (gamma rays). Most of the sources in Junior Lab emit only gamma radiation. Of the sources which do emit alpha or beta particles, most are enclosed in plastic or metals, which prevent particulate radiation from escaping. The exceptions are the ⁹⁰Sr source in the e/m experiment and the ²⁴¹Am source in the Rutherford Scattering experiment; both sources are in an enclosed apparatus. These sources should never be handled. Handling of open alpha- or beta-emitters can result in dangerous dosages to the skin.

The strength of a radioactive source is measured in curies (Ci). A one-curie source has an activity of 3.7 x 10¹⁰ disintegrations s^-1. The "absorbed dose" is a quantity that measures the total energy absorbed per unit mass; it is measured in rads, where 1 rad = 100 erg g^-1. The "equivalent dose" is measured in the units discussed above, the rem. The equivalent dose is derived from the absorbed dose by multiplying by a "radiation weighting factor" which is a measure of how damaging a particular type of radiation is to biological tissue. For photons (gamma rays) and electrons and positrons (beta particles), the radiation weighting factor is unity; for helium nuclei (alpha particles), it is 20; for protons with energy greater than 2 MeV it is 5; and for neutrons it ranges from 5 to 20, depending on the energy. When you use the meter in the lab, the readings are in rads, and you must consider the type of particle when you work out the equivalent dose.

For gamma rays with energy greater than 1 MeV, a useful approximation is that the equivalent dose due to a source with an activity of C microcuries is 5.2 x 10^-4CE_γR^-2 mrem hr^-1, where R is the distance from the source in meters and E_γ is the energy of the gamma ray in MeV. For gamma rays with energy less than 1 MeV, this formula is still approximately true for a fullbody dose. However, low-energy gamma rays deposit their energy in a smaller mass of tissue than high-energy gamma rays and can cause high local doses. For example, the local dose to the hands from handling a 10 KeV source can be up to 25 times the value given by the above formula; hands, however, have a higher tolerance to radiation than inner organs or eyes.

The protective value of shielding varies drastically with the energy of the photons. The intensity of a soft X-ray beam of "soft" (i.e. < 1 KeV) can be reduced by many orders of magnitude with a millimeter of aluminum while 1.2 MeV gamma rays from ⁶⁰ Co are attenuated by only a factor of 2 by a lead sheet one-half of an inch thick. The best way to keep your dosage down is to put distance between you and the source. If you stay a meter away from most sources in Junior Lab, you will be receiving, even without any lead shielding, a dose which is much less than your allowable background dose. If, however, you sit reading the write-up with a box of sources a few inches away, you may momentarily be receiving ten to a hundred times the background level.

List of Precautions for Working with Radioactive Materials

1. Don't handle radioactive sources any more than you have to.
   
   
2. Work quickly when transferring or positioning radioactive sources.
   
   
3. Never take a source away from the Junior Lab, even temporarily. The senior staff are legally responsible for the sources and must periodically account for their presence and condition.
   
   
4. Replace sources in the lead storage cabinet when they are not in use and ensure that the cabinet is locked at all times.
   
   
5. Keep sources away from your body.
   
   
6. Never bring a radioactive source near your eyes because they are particularly sensitive to radiation.
   
   
7. Be aware of the sources being used in neighboring experiments.

A table showing the radioactive sources used in Junior Lab and their approximate activities is given below.