Syllabus

Goals

The purpose of Junior Lab is 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 hope 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

The class is divided into three sections per week. Sections are

Monday / Wednesday 1:30-4:30 pm
Tuesday / Thursday 9:00-12:00 pm
Tuesday / Thursday 2:00-5:00 pm

The introductory sessions will be dedicated to selecting experimental lines. Students will choose 5 different experiments, the first 2 of which will be performed over 5 separate lab sessions and the last 3 of which will be performed over 4 separate lab sessions. This will leave 1 free session at the end of the term to help students catch up.

New this term will be the possibility of performing an experiment for a 'double' session in which one of the 4-session blocks can be dedicated to improving an existing experiment or by extending your investigations beyond the lab guide. Permission to perform a 'double' experiment must be received from your section instructor but will require the submission of a <1 page proposal describing what you intend to do during the additional 4 sessions. In grading this 'double' session, only ONE written paper will be required, albeit longer than a normal summary, for the combined 'double' session. A single oral exam will be conducted as usual, after the completion of the FIRST normal block (normally 4 or 5 sessions). Note: Your decision to perform or not, a 'double' session will have NO bearing on your final grade.

Each section is run independently with the assistance of a graduate teaching assistant and an undergraduate teaching assistant.

You are expected to work in pairs, sharing as evenly as possible in the setup and operation of the equipment, the analysis, and interpretation of the data. The best choice for lab partner may be someone who lives nearby and has a schedule that matches yours so you can get together easily outside of class to prepare for your lab sessions and to analyze and interpret your results. Most students find they require at least the full 18 hours per week credited to Junior Lab to do the work of the course. If you have an overload, consider lightening it up so as to leave adequate time to get the full benefit of this course.
 
Ethics in Science and Science Education

When you read the report of a physics experiment in a reputable journal (e.g. Physical Review Letters) 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 and errors they publish.

Junior Lab is designed to provide preprofessional training in the art and science of experimental physics. In keeping with the spirit of trust in science Junior Lab instructors will assume that what you record in your lab book and report in your written and oral presentations is exactly what you have observed. Sometimes you'll get things wrong because of an error in manipulation, equipment malfunction, misunderstanding, or a miscalculation. Try to evaluate these before the last session so the instructor can 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, or copying from "the files" are intellectual crimes as serious as plagiarism, and possible causes for dismissal from the Institute.

The acknowledgement of other people's data also applies to the use of 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.

If you do choose to quote material, it is not sufficient just to include the original source among the list of references at the end of your paper. If a few sentences or more are imported from another source, that section should be indented on both sides or enclosed in quotes, and attribution must be given immediately in the form of a footnote.

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.

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.

Practical Hints

Most Junior Lab experiments are concerned with 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 the manipulation of 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 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 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 in a result, whether it be 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.

Attendance is Mandatory

In order to get your tuition's worth (~$100 per 3-hour lab session) it is essential that you use efficiently all the laboratory time assigned to you, and, if necessary, more (Fridays). An experienced experimental physicist will be present in every scheduled session. He or she will be assisted by a graduate teaching assistant and also by an undergraduate teaching assistant who took the course last year. In addition, the Junior Lab staff includes two technical instructors responsible for the maintenance of the equipment and the development of new experiments. The teaching and technical staff are ready and eager to help you make things work properly and answer questions. Call on them for help when you get stuck.

You are required to attend each of your assigned lab sessions for the full 3-hour period. Any exceptions must be negotiated with the professor in charge of your section. The regularity of your attendance will be a factor in determining your grade in the course.

The laboratories will be open every weekday from 9 to 5 (except Institute holidays) with staff help available to teach physics and maintain equipment. At all other times the laboratories must be kept locked to prevent theft. Junior Lab students may occasionally be permitted access to the lab outside of the normal hours, but only after consulting with their TA or faculty member. It is each student's responsibility to maintain security by making sure the doors are kept locked at all times outside of the regularly scheduled sessions.

Laboratory Notebook

Your lab notebook is the critical proof of your efforts in your research. Therefore each date and collaborator MUST be noted as well as a clear trace of your quoted results. A principal objective of this course is to acquaint you with the systematics of experimentation and record keeping that will serve you well in future research. To this end you will be given a standard Computation Book 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. Each student must keep a complete, dated record of each experiment and its analysis, although you will generally work in pairs and are urged to collaborate in all aspects of carrying out the experiments and analyses, 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 photocopies 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 if necessary reconstruct all the significant details of what you did and 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. The quality of your lab notebook will be an important factor in the evaluation of your performance in the course.

Analyze data in the lab in at least a preliminary way as you go along to check for reasonableness. If you are making 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 points. 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 (or five) regularly assigned lab sessions, take the opportunity to perfect part or all of the experiment so as to obtain the best possible data set.

Some 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 by hand to ensure that the subsequent data are correct. 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.

Preparatory Questions

Each lab guide has a set of preparatory questions. 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 the designated box. It will be collected shortly after the start of the first session. Late solutions will not be accepted. Your solutions will be graded by the graduate teaching assistant and returned at the next session.

First Session

The first lab session will meet at the scheduled times and will be devoted to introductory remarks and organizational matters. During the first session you will have an opportunity to explore the laboratory, peruse the lab guides, and choose a partner. An important task during the first session is to choose experiment "lines" (i.e. set of 5 experiments) to each team of students. This is done in a semi-lottery fashion. Regular sessions will begin with the second scheduled sessions.

Oral Presentations

A one-hour oral review and discussion of each experiment will be held between the pair of students and one or more of their instructors within two weeks of the last scheduled session for that experiment. Each student is expected to bring his or her lab notebook to the examination. Each student should prepare a 15-minute oral report on the theoretical and experimental aspects of a single portion of the experiment.

We strongly encourage you to make computer-based presentations on Athena using the quick start guide to Creating LaTeX Slide Presentations (PDF). We also provide you with a Sample LaTeX Presentation File (PDF) you can modify and customize for your own presentations. If overhead projector is used, blank transparencies (not more than 12 per talk) are available.

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 Physical Society. 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 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 theory and experiment; it is not acceptable to discuss theory only or experiment only. Full cooperation with lab partners and others in preparing the material for the written summaries and the oral reports is encouraged. The final words however must be your own!!!

Written Summaries

By midnight of the day of your oral examination, you must email an Adobe^® Acrobat^® PDF copy of your (<4) page, individually-prepared, written summary of the purpose, theory, and results of the experiment. The delay between oral exam and paper submission is designed to allow 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 (PDF) that we encourage you to study and use for your own submissions.

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 in-line within the text and the sample paper template (PDF) 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 according to the following scheme: Motivation - 10%, Experiment - 40%, Analysis - 30%, Style and English - 20%.

After the oral examination, performance evaluations will be recorded and reported to the students by email or otherwise, according to the section instructor's preference.

Grades

Your grade in 8.14 will be based 10% on your written answers to the preparatory problems, 50% on the oral examinations and 25% on the written summaries. The quality of your lab notebook, and the general impression of your lab performance including attendance will account for the remaining 15% of your grade.

Required Texts

Melissinos, Adrian. Experiments in Modern Physics. Academic Press, 1966. Material which can be found in the Melissinos text is generally omitted from the Lab Guide.

Bevington, Philip R. Data Reduction and Error Analysis for the Physical Sciences. McGraw-Hill. This book contains a comprehensive treatment of error analysis and numerous algorithms that will be useful in your future research career.

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.

1. High Voltage

The first rule is never to work alone. Some years ago a student was electrocuted 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.

2. Lasers

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.

3. Cryogenics

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.

4. Radiation

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 use 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.

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.

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 millirems/year, which works out to 4.2x10^-2 millirems/hour. The recommended limit to exposure for a member of the general public is 100 millirem/year, averaged over any consecutive five years. As a Junior Lab student you receive instruction in the use of radioactive materials, and you work in an area that is clearly marked as one in which radioactive materials are in use. This means you are considered a radiation worker, and the recommended occupational dose limit (whole-body) in the United States is 5 rem/year, though many labs in the US and elsewhere set lower limits. The International Commission on Radiation Protection (ICRP) recommended in 1991 a limit of 2 rem/year averaged over 5 years with the dose in any one year not to exceed 5 rem. If you follow the Junior Lab guidelines, your exposure will be only a small fraction of the dose you receive from the natural background. The Table lists the radioactive sources in use in Junior Lab, along with their activities as measured within the past two years. A meter is available for you to check the levels yourself.

Radioactive Sources in Use in Junior Lab

EXPERIMENT ISOTOPE APPROXIMATE ACTIVITY (mCi) Compton Scattering 137Cs 0.4 Relativistic Dynamics 90Sr 8 133Ba 0.08 Mossbauer Spectroscopy 57Co 7 Rutherford Scattering 241Am 0.2 X-Ray Physics 241Am 10 55Fe 0.7 90Sr 0.6 57Co 0.02 Alpha Decay U ore 5 x 10-6 Calibration 133Ba 0.005 109Cd 0.008 137Cs 0.007 57Co 0.0001 60Co 0.0005 54Mn 0.0002 22Na 0.002

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The strength of a radioactive source is measured in curies (Ci). A one-curie source has an activity of 3.7 x 10¹⁰ disintegrations per second. 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/gm. 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 particales), the radiation weighting factor is unity; for helium nuclei (alpha particles), it is 20; for protons with energy greater than 2Mev 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^-4 CE_gamma/R² millirem/hr, where R is the distance from the source in meters and E_gamma 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 full-body 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 ⁶⁰Co 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.

We end with a list of precautions:

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.
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.

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