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Explaining the Composition of the Present-Day Universe

Todd Brown
Physics Department
Vanderbilt University
August 2001

My study is a portion of an ongoing work being performed by numerous worldwide laboratories attempting to explain the abundance of materials seen in the Universe today. Actually, these materials are elements in which the distinction between different elements is the number of protons (small, positively charged particles found at the centers of atoms) that each contains. For example, hydrogen atoms always contain one proton whereas helium atoms always have two. When complete, it is hoped that scientists will be able to understand why certain elements are rarer than others and how the Universe was able to produce all elements that make up the stars, the planets and even ourselves.

When science talks about the "formation of the Universe", the Big Bang Theory come to mind. Although the details of this theory are not important here, it is useful to know that it states that the early Universe was made of only the five lightest elements. The heavier elements, most of which make up the earth and living organisms, did not exist in the youthful Universe. Therefore, they had to be created using these lighter five elements. This process of building heavier elements from lighter ones, termed nucleosynthesis, can only be accomplished in the dense, high-temperature environments of either stellar cores or in the explosive deaths of extremely massive stars (supernovae).

Heavier elements are made by combining lighter elements, analogous to a stone wall being made by combining single pieces of rock. A more detailed look shows that the quantities of various types of "building blocks" used in today's Universe are not equal (i.e. the proportion of the "stone wall" made of carbon do not equal the proportion made of gold and this is seen by the fact that carbon is more common than gold). Also, each element is actually a mixture of varieties differing in the number of neutrons, a particle found in atomic centers. These varieties, termed "isotopes", also come in various amounts which must be explained by any theory that deals with nucleosynthesis. For example, a sample of carbon from a piece of wood is mainly Carbon-12 (atoms with 6 neutrons and 6 protons). A smaller percent of this sample will be Carbon-13 (atoms with 7 neutrons and 6 protons). Why such differences are seen is the heart of the question that fueled my research.

My focus was on an isotope of Selenium (abbreviated 'Se') which had 34 protons and 34 neutrons: 68Se (where 68 refers to the total number of protons and neutrons found in this isotope). Rarely found in nature due to the fact that it is radioactive, it was theorized to be a key bridge to build heavier elements during a supernova explosion. Astrophysicists needed to know its half-life and mass to determine if it could survive long enough for it to be used to produce heavier elements in a supernova.

The task comes in producing these isotopes which are normally seen only for a short time and under the conditions of a supernova. To create 68Se on earth, a device called an accelerator is used. An accelerator can be thought as being similar to a flashlight battery. In a battery, one end is at ground (0 Volts) and the other end might be at 3 volts. This is termed a "potential difference" and, once both ends are connected by a wire, allows electrons to flow. With an accelarator, the potential difference is far greater. So much, in fact, that whole atoms are moved, not just electrons. The goal is to get the atoms moving fast enough so that when they encounter a target atom they collide and merge.

In my experiment, an accelerated beam of nickel atoms impacted onto a target of carbon atoms. This merging is just the initial step towards the possible production of 68Se. The analogy of this first step is two cars colliding where one car is a nickel atom and another is a carbon atom. Different-sized pieces (a taillight, a bumper, etc) might break off but, for a successful collision, the main bodies of the cars must stick together (merge). Different collisions under nearly the same conditions might end differently (i.e. a sidemirror might break off one time and not on the second time). This was my challenge. I was looking for a specific type of outcome (68Se). Sometimes the collision formed it, usually it formed something else. With powerful computers to enable the meager amounts of 68Se produce to be sifted from an immense background of static, it was possible to measure the half-life of 68Se (38.8 ~ 2.6 seconds) and its mass. Both met theoretical expectations and allowed my team to confidently state that 68Se, although it does not exist long in nature, does act as a key stepping stone to the creation of heavier elements that are seen in today's world.