September 30 is Hans Geiger’s birthday. Geiger was the German physicist best known for the invention of the Geiger counter and the Geiger-Marsden experiment.
The Geiger-Marsden experiment is more commonly known as the “Gold foil experiment” that demonstrated the existence of the atomic nucleus. The prevailing theory dealing with the structure of the atom was known as the ‘plum pudding’ model. The atom was a collection of electrons distributed in a region of positive charge. These electrons were distributed much like the plums in a plum pudding made of positive charge: in there, but not ordered in any way.
Ernest Rutherford arranged for Hans Geiger and Ernest Marsden to test this theory. Their idea was to fire positively charged alpha particles towards a thin foil of gold. If the plum pudding model was true, the alpha particles would be deflected away from the atoms overall positive charge. Because the electric field produced by single atoms is small, the deflection of the alpha particle should also be small. In effect, alpha particles should appear to pass straight through a sample of atoms. When they fired their beam of alpha particles through the gold foil, they found the particles mostly traveled straight through the sample, a significant number of them were being scattered all over the place. Clearly, the plum pudding model was incorrect. They could only get these path deflections if the atom’s positive charge was more concentrated within the atom or a nucleus of charge.
One side issue to the gold foil experiment was the need to detect alpha particles. One solution to this problem was the invention of the Geiger counter. Geiger counters are radiation detectors. The basic set-up is a sealed metal tube with a wire running down the center axis. The tube is filled with an inert mixture of gasses. When an electric charge is applied to the central wire, an electric field forms between the wire and inside edge of the tube. When a particle or photon of radiation enters the tube, the gas ionizes and becomes conductive. This causes a change in the tube’s electric field and produces a small current on the wire until the gas reverts to its stable form. This current spike is recorded by the device. Many Geiger counters have displays that show the number of current spikes per unit of time but most early models only sent the current spike to a speaker. The current causes the speaker to click when radiation is detected. The more clicks, the more radiation encountered. in the form of a counter, audible signal to show it has detected radiation. The sensitivity of the device can be changed by the initial voltage applied to the central wire.
Geiger counters can easily be purchased from scientific supply houses or even Amazon.com. The prices range from $50 to $1000, depending on the options, sensitivity, and particles it can detect. Using one can be a surprising adventure when you find radioactivity in places you never suspected before.
Notable Science Events for September 30
2014 – Martin Lewis Perl died.
Perl is an American physicist who was awarded half the 1995 Nobel Prize in Physics for the discovery of the tau lepton. The tau lepton is an extremely short lived (2.9 x 10-13 seconds) negatively charged elementary particle. Tau leptons have 3477 times the mass of electrons. The particle was detected during collisions between positrons and electrons using the SPEAR (Stanford Positron Electron Asymmetric Rings) particle accelerator. This accelerator could build up particle energies to 4.8 GeV, high enough to create tau particles.
1994 – André Michel Lwoff died.
Lwoff was a French microbiologist who was awarded a third of the 1965 Nobel Prize in Medicine for the discovery that lysogenic bacteria (bacteria that is infected by viruses) remain lysogenic in future generations. This showed the bacteriophage genetically insert themselves into the bacteria and reproducing itself. He also found a non-infecting bacteriophage that he called prophage that inhibited virus reproduction. The prophage was later found to become active when exposed to ultraviolet light which gave evidence to cancerous tumors could be caused by mutations of an inactive or dormant virus.
1985 – Charles Richter died.
Richter was an American seismologist who is best known for the logarithmic earthquake magnitude scale that bears his name. When an earthquake occurs, the maximum amplitude of the shake is measured on a seismometer and assigned a Richter number. A quake with a value of 5 on the Richter scale is 10 times more powerful than a quake with a value of 4.
1951 – Barry James Marshall was born.
Marshall is an Australian physician who shares the 2005 Nobel Prize in Medicine with J. Robin Warren for the discovery of the Helicobacter pylori bacterium. He showed this bacteria is the main cause of most peptic ulcers and not things like stress or spicy food.
1943 – Johann Deisenhofer was born.
Deisenhofer is a German biochemist who shares the 1988 Nobel Prize in Chemistry with Robert Huber and Hartmut Michel for the determination of the structure of the proteins essential for photosynthesis. They used x-ray crystallography to determine the structure of the protein and greatly increased the understanding of the mechanism of photosynthesis of plants and bacteria.
1939 – Jean-Marie Lehn was born.
Lehn is a French chemist who shares with Jean-Marie Lehn and Charles Pedersen the 1987 Nobel Prize in Chemistry for their research and development of host-guest chemistry. Host-guest chemistry is where two or more molecules/ions bond in unique ways due to their structure in other than covalent bonds. Lehn created molecules that acted as a cage molecule where a central molecule or atom would get trapped within a cavity created by the cage molecule.
1905 – Nevill Francis Mott was born.
Mott was an English physicist who shared the 1977 Nobel Prize in Physics with Philip W. Anderson and J. H. Van Vleck for their independent work with the electronic structure of magnetic and disordered systems. A disordered system is a solid with no long range order to the group of molecules that make up the solid. Glass is an example of a disordered system.
1882 – Hans Geiger was born.
1870 – Jean Perrin was born.
Perrin was a French physicist who discovered cathode rays were actually made up of corpuscular negative charges (electrons). He is best known for verifying Einstein’s theories of Brownian motion and calculating Avogadro’s number, the number of molecules per mole of a gas. His work in determining the equilibrium constant of suspended solutions would earn him the 1926 Nobel Prize in Physics.
1802 – Antoine Jérôme Balard was born.
Balard was a French chemist who isolated and identified the element bromine. He was conducting a general investigation of seawater when he found the previously unknown element in seaweed and several marine animals.
1694 – Marcello Malpighi died.
Malpighi was an Italian physician who is considered the father of microscopic anatomy and histology. He also used the microscope to study the development of chick embryos and insects do not have lungs, but breathe through tracheae holes in their skin.