PHYSICS 111 LAB Physics Department Experiment 9
Total Page：16
File Type：pdf, Size：1020Kb
PRINCETON UNIVERSITY PHYSICS 111 LAB Physics Department Experiment 9 Radioactivity ABSOLUTELY NO EATING OR DRINKING IN THE LAB 1. Background Information 1.1 Radioactivity The atoms making up matter are generally stable, but some of them are spontaneously transformed by emitting radiation (and therefore energy). This is called radioactivity. It was first discovered by Henri Becquerel in 1896, who showed that the radiation spontaneously emitted by Uranium made an imprint on photographic film and made air conduct electricity. Pierre and Marie Curie discovered two other elements, polonium and radium, that emitted similar radiations. The three shared the Nobel Prize in Physics in 1903 for their discoveries. Atomic nuclei are composed of protons and neutrons, which are in turn made up of quarks. Isotopes of a given element contain the same number of protons (and therefore electrons), but differ in the number of neutrons. Heavy, unstable nuclei are transformed into lighter, stable nuclei by radioactive emission. There are three types of radiation, corresponding to three types of radioactivity: • Alpha radioactivity corresponds to the emission of a helium nucleus (two protons + two neutrons), called an alpha-particle. • Beta radioactivity corresponds to the transformation in the nucleus of i) a neutron into a proton and an electron (β- radioactivity) or ii) a proton into a neutron and an anti- electron or positron (β+ radioactivity). This latter type of beta radioactivity appears only in artificial radioactive nuclei produced by nuclear reactions. • Gamma radioactivity unlike the other two types does not correspond to a transformation from one element to another. Rather, it results in the emission by the nucleus of electromagnetic radiation. This type of radioactivity can occur by itself, or together with alpha or beta radioactivity. Alpha and beta radiations, as well as electromagnetic radiation such as gamma rays and X-rays and some ultraviolet rays are said to be ionizing. That is, they have enough energy to eject one or more electrons from atoms or molecules in the irradiated medium. The activity of a radioactive body is measured in Becquerels: One Becquerel corresponds to the disintegration of one atomic nucleus per second. As unstable atoms are transformed, the radioactivity of the sample containing them decreases. The time it takes for this activity to decrease by half is called the half-life. The half-life is characteristic of a particular isotope and may range from fractions of a second to billions of years. For example: Polonium-214 0.164 s Oxygen-15 2 minutes Carbon-14 5730 years Uranium-238 4.5 billion years Natural radioactivity has been present since the beginning of the Universe, as radioactive atoms undergo series of transformations, which convert them to the final stable nuclei. The natural sources of radiation on Earth are mostly radium, uranium and thorium, present in soil and rocks, as well as radon gas produced from the decay of uranium in soil. Artificial (man-made) radioactivity is the same phenomenon as natural radioactivity, however the emitting nuclei are produced in the lab by bombarding an element with alpha-particles, for example. Some measures of background radioactivity are: 1 liter of milk ~ 60 Bq 5-yr old child ~ 600 Bq 70 kg adult ~ 10,000 Bq (Potassium-40 is one source of radioactivity in the human body). 1 g radium ~ 37 billion Bq Another unit of radioactive decay is the Curie. One curie equals 3.7×1010 radioactive decays per second. So one curie is a lot of radiation, and 1 Becquerel is not very much. Note that measurements in Curies or Becquerels do not tell you what sort of radiation is involved, or how much energy is carried by the emitted particles. They only tell you how many particles are emitted, on average, in a given amount of time. (How do these compare with the activity of your radioactive sources?) 1.2 Cosmic Rays At the start of the 20th century, scientists were puzzled by the fact that there seemed to be more radiation in the environment than they could account for from known sources of natural background radioactivity. The puzzle was solved in 1912 by a German scientist, Victor Hess (Nobel Prize 1936), who went up in a hot-air balloon to 17,500 ft (without oxygen) and observed (using a gold-leaf electroscope) that the amount of radiation increased as his balloon climbed, despite the fact his distance to radiation sources on Earth was increasing. Thus, it was determined that the radiation was from outer space, and it was dubbed “Cosmic Rays”. We know from measurements made on board satellites and high altitude balloons that the vast majority of cosmic rays are protons (nuclei of hydrogen atoms). Other heavier atomic nuclei, extending all the way up to uranium, as well as electrons, are also present. A very small fraction (0.1 percent) of cosmic rays are photons (in the form of gamma rays). Some of the particles comprising cosmic rays originate from the Sun, but most come from sources outside the solar system and are known as galactic cosmic rays (GCRs). Cosmic ray particles that arrive at the top of the Earth’s atmosphere are called primaries; their collisions with atmospheric nuclei give rise to other particles known as secondaries. From the early 1930s to the 1950s, before the advent of powerful particle accelerators, cosmic rays played a critical role in studying the atomic nucleus and its components, for they were the only source of high- energy particles. Short-lived subatomic particles were discovered through cosmic ray collisions (pions and muons). At present, the most energetic cosmic rays contain particles with energies far beyond those attainable under laboratory conditions. Today cosmic rays are studied to learn about their sources, the mechanism by which they are accelerated to such high energies and the distribution of matter traversed by them - they are one of the few things we can measure which originate outside our own solar system. 1.3 Wilson’s Cloud Chamber In 1927, Charles Thomson Rees Wilson was co-recipient of the Nobel Prize in Physics for his discovery of a method for making visible the tracks followed by electrically charged particles. Wilson’s method was based on the formation of clouds, which develop when sufficiently moist air is suddenly expanded. The refrigeration caused by the expansion causes the temperature to sink below the dew-point (supercooling), and the vapor is condensed into small drops, which form together visible clouds. In the first stage of condensation, a droplet is formed around a “nucleus”. Electrically charged particles can act as a nucleus in the formation of drops, as they attract other nearby atoms. Ionizing radiation from radioactive sources or cosmic rays, when traveling through a container of supercooled gas, leaves behind a track of charged ions, which is marked by the formation of droplets. Some of the important achievements using the Wilson chamber were the discovery of the positron (anti-matter partner of the electron, positively charged), and demonstration of the recoil of an electron and an X-ray photon. 2. The Cloud Chamber 2.1 Setting it up The chamber consists of a glass (or plastic) dish with a square glass top. The dish rests on a flat bed of finely chopped dry ice. Do not press the dish to force contact with the ice. Simply let the dish rest on top of the ice layer. The dry ice is spread over the insulating acoustic tile. A spot-light is positioned to one side so that the strong white light will illuminate the cloud tracks formed by the radioactive particles. Be careful not to “burn” yourself with the dry ice. Take a handful of it (using the gloves) and spread it evenly over the tile, about 1 cm deep. Lay the glass dish on top of the ice, and pour in the contents of the alcohol bottle. This dye will provide a dark background. Make sure the bottom of the dish is covered uniformly with the alcohol. Lay the glass plate over the top of the dish, be sure that the light from the lamp passes through the heat-absorbing glass which fits in the wooden stand. Adjust the cardboard light shield so that the light only passes into the vessel and does not shine on the top glass plate. Your final set-up looks like the figure below. 2.2 How it works The methanol in the dish bottom is wicked up the sides by the felt cloth and evaporated. As the evaporated methanol approaches the cold surface of the methanol on the bottom, it becomes supercooled and wants to condense. It will do so around nucleation centers, such as dust, or in our case, ions charged by the passage of ionizing radiation. When a charged particle passes through the methanol vapor, it leaves a trail of ions and electrons. The methanol condenses around the ions, forming a visible trail. (Note that the supercooling in our chamber is not achieved through expansion of the gas, as in the Wilson chamber). 2.3 Track-Watching Without a radioactive source Are you able to see these cosmic ray tracks? e µ ν ν • A track which goes straight, then kinks off to the left or right sharply is a “muon” decay. A muon is a “heavy cousin” of an electron, and decays into an electron (which leaves a track in the chamber) and neutrinos (which do not leave tracks in the chamber because they do not interact with the gas). µ µ e • Three tracks meeting at a single point correspond to an incoming cosmic ray (one track), which hits an atomic electron, with the electron and the outgoing cosmic track making up the two other tracks.