r0 r1 r3 r4 r5 In physics, induced gamma emission (IGE) refers to the process of fluorescent emission of gamma rays from excited nuclei, usually involving a specific nuclear isomer. It is analogous to conventional fluorescence, which is defined as the emission of a photon (unit of light) by an excited electron in an atom or molecule. In the case of IGE, nuclear isomers can store significant amounts of excitation energy for times long enough for them to serve as nuclear fluorescent materials. There are over 800 known nuclear isomers but almost all are too intrinsically radioactive to be considered for applications. As of 2006 there were five proposed nuclear isomers that appeared to be physically capable of IGE fluorescence in safe arrangements: tantalum-180m, osmium-187m, platinum-186m, hafnium-178m2 and zinc-66m.

Contents 1 History 2 Distinctive features 3 Potential applications 3.1 Energy-specific dosimeters 3.2 Aircraft power 3.3 Fusion bomb ignition 4 See also 5 References 6 Literature 7 External links

History Energetics of IGE from 115In. Arrows are photons, (up) absorption, (down) emission. Horizontal lines represent excited states of In involved in IGE.

Induced gamma emission is an example of interdisciplinary research bordering on both nuclear physics and quantum electronics. Viewed as a nuclear reaction it would belong to a class in which only photons were involved in creating and destroying states of nuclear excitation. It is a class usually overlooked in traditional discussions. In 1939 Pontecorvo and Lazard reported the first example of this type of reaction. Indium was the target and in modern terminology describing nuclear reactions it would be written 115In(γ,γ')115mIn. The product nuclide carries an "m" to denote that it has a long enough half life (4.5 hr in this case) to qualify as being a nuclear isomer. That is what made the experiment possible in 1939 because the researchers had hours to remove the products from the irradiating environment and then to study them in a more appropriate location.

With projectile photons, momentum and energy can be conserved only if the incident photon, X-ray or gamma, has precisely the energy corresponding to the difference in energy between the initial state of the target nucleus and some excited state that is not too different in terms of quantum properties such as spin. There is no threshold behavior and the incident projectile disappears and its energy is transferred into internal excitation of the target nucleus. It is a resonant process that is uncommon in nuclear reactions but normal in the excitation of fluorescence at the atomic level. Only as recently as 1988 was the resonant nature of this type of reaction finally proven. Such resonant reactions are more readily described by the formalities of atomic fluorescence and further development was facilitated by an interdisciplinary approach of IGE.

There is little conceptual difference in an IGE experiment when the target is a nuclear isomer. Such a reaction as mX(γ,γ')X where mX is one of the five candidates listed above, is only different because there are lower energy states for the product nuclide to enter after the reaction than there were at the start. Practical difficulties arise from the need to ensure safety from the spontaneous radioactive decay of nuclear isomers in quantities sufficient for experimentation. Lifetimes must be long enough that doses from the spontaneous decay from the targets always remain within safe limits. In 1988 Collins and coworkers reported the first excitation of IGE from a nuclear isomer. They excited fluorescence from the nuclear isomer tantalum-180m with x-rays produced by an external beam radiotherapy linac. Results were surprising and considered to be controversial until the resonant states excited in the target were identified. Fully independent confirmation was reported by the Stuttgart Nuclear Group in 1999. Distinctive features If an incident photon is absorbed by an initial state of a target nucleus, that nucleus will be raised to a more energetic state of excitation. If that state can radiate its energy only during a transition back to the initial state, the result is a scattering process as seen in the schematic figure. That is not an example of IGE. If an incident photon is absorbed by an initial state of a target nucleus, that nucleus will be raised to a more energetic state of excitation. If there is a nonzero probability that sometimes that state will start a cascade of transitions as shown in the schematic, that state has been called a "gateway state" or "trigger level" or "intermediate state". One or more fluorescent photons are emitted, often with different delays after the initial absorption and the process is an example of IGE. If the initial state of the target nucleus is its ground (lowest energy) state, then the fluorescent photons will have less energy than that of the incident photon (as seen in the schematic figure). Since the scattering channel is usually the strongest, it can "blind" the instruments being used to detect the fluorescence and early experiments preferred to study IGE by pulsing the source of incident photons while detectors were gated off and then concentrating upon any delayed photons of fluorescence when the instruments could be safely turned back on. If the initial state of the target nucleus is a nuclear isomer (starting with more energy than the ground) it can also support IGE. However in that case the schematic diagram is not simply the example seen for 115In but read from right to left with the arrows turned the other way. Such a "reversal" would require simultaneous (to within <0.25 ns) absorption of two incident photons of different energies to get from the 4 hr isomer back up to the "gateway state". Usually the study of IGE from a ground state to an isomer of the same nucleus teaches little about how the same isomer would perform if used as the initial state for IGE. In order to support IGE an energy for an incident photon would have to be found that would "match" the energy needed to reach some other gateway state not shown in the schematic that could launch its own cascade down to the ground state. If the target is a nuclear isomer storing a considerable amount of energy then IGE might produce a cascade that contains a transition that emits a photon with more energy than that of the incident photon. This would be the nuclear analog of upconversion in laser physics. If the target is a nuclear isomer storing a considerable amount of energy then IGE might produce a cascade through a pair of excited states whose lifetimes are "inverted" so that in a collection of such nuclei, population would build up in the longer lived upper level while emptying rapidly from the shorter lived lower member of the pair. The resulting inversion of population might support some form of coherent emission analogous to amplified spontaneous emission (ASE) in laser physics. If the physical dimensions of the collection of target isomer nuclei were long and thin, then a sort of "gamma ray laser" might result. Potential applications Energy-specific dosimeters

Since the IGE from ground state nuclei requires the absorption of very specific photon energies to produce delayed fluorescent photons that are easily counted, there is the possibility to construct energy-specific dosimeters by combining several different nuclides. This was demonstrated for the calibration of the radiation spectrum from the DNA-PITHON pulsed nuclear simulator. Such a dosimeter could be useful in radiation therapy where X-ray beams may contain many energies. Since photons of different energies deposit their effects at different depths in the tissue being treated, it could help calibrate how much of the total dose would be deposited in the actual target volume. hafnium crystalline bar Aircraft power

In February 2003, the non-peer reviewed New Scientist wrote about the possibility of an IGE-powered airplane, a variant on nuclear propulsion. The idea was to utilize 178m2Hf (presumably due to its high energy to weight ratio) which would be triggered to release gamma rays that would heat air in a chamber for jet propulsion. This power source is described as a "quantum nucleonic reactor", although it is not clear if this name exists only in reference to the New Scientist article. Fusion bomb ignition

It is partly this theoretical density that has made the entire IGE field so controversial. It has been suggested that the materials might be constructed to allow all of the stored energy to be released very quickly in a "burst". The density of gammas produced in this reaction would be high enough that it might allow them to be used to compress the fusion fuel of a fusion bomb. If this turns out to be the case, it might allow a fusion bomb to be constructed with no fissile material inside (i.e. a pure fusion weapon), and it is the control of the fissile material and the means for making it that underlies most attempts to stop nuclear proliferation. In fact, the possible energy release of the gammas alone would make IGE a potential high power "explosive" on its own, or a potential radiological weapon. See also Hafnium controversy, use of stored energy in Hf-178m2 Laser, induced light emission

Carlo Orelli and Induced gamma emission

Carlo Orelli (December 23, 1894 – January 22, 2005) was, at age 110, the last surviving Italian World War I veteran who joined the army at the onset of the war. Born in Perugia, although he lived in Rome for most of his life, Orelli came from a military family whose members had served in various Italian conflicts since 1849. A mechanic by trade, Orelli joined the Italian Army in May 1915 and engaged in combat operations in Italy. His recollections were marked by particularly brutal experiences of trench warfare, including the violent deaths of many of his friends. After receiving injuries to his leg, he was pulled from active duty and returned home.

After recovering from a related infection, Orelli married and had a family of six children. During World War II, despite his aversion to Nazi Germany and fascism, he was forced to work as an artillery director in Italy. At war's end, he returned to work as a mechanic, retiring in 1960. In his later years, he was active in urging others not to forget the lessons learned after the first World War, and in 2003 he was made a Grand Officer in the Order of Merit of the Italian Republic. He died in January 2005, a month after having turned 110 and achieving supercentenarian status. At the time of his death he was Italy's oldest survivor of the First World War, the last remaining trench infantryman and the last survivor from Italy's entry into the war.

Contents 1 Early life 2 Military career 3 Post-World War I 4 See also 5 References 6 Further reading

Early life

Orelli was born in Perugia on December 23, 1894, although his family soon moved to Rome to be nearer to an aunt that operated a local tavern. Carlo came from a long history of military activity. His maternal grandfather, Thomas, helped defend Perugia against Austrian mercenaries in 1849, while Carlo's father served in the Italian Abyssinian campaign during the Scramble for Africa in the 1880s. His elder brother Alfredo fought in Libya during the Italo-Turkish War in 1911, while his younger brother William fought in World War II and was captured by the British in July 1943. He lived in the Garbatella district, one of the most secluded areas of Rome, on the fourth floor of a house without an elevator. Before joining the war, he trained to become a mechanic. Military career

Orelli signed up for active duty at the age of 21 and joined the Austro-Hungarian front at the onset of the war in May 1915. Entrenched with the 320 Infantry Regiment, he served as a foot soldier within the Italian Army and engaged in combat activity in the trenches near Trieste, which was the main battleground in the east. He also participated in combat operations around the Isonzo river. In his autobiography, Orelli went into great detail about his experiences, including the large amount of casualties that he witnessed during his time in the trenches and widespread illiteracy among the peasant soldiers. He referred to them as having "died in silence." He also recalled many violent deaths, such as a friend whose feet were severed and subsequently bled to death and another who was decapitated in mid-conversation with Orelli. He remarked of episodes such as this that they were "things I do not wish to remember."

In interviews, he also described the experiences of fear that were common among the soldiers, and lamented the way in which films portrayed combat experiences. When it came to cannon shells, for example, the impact was not immediate, but approached with a gradual and frightful sound. He referred to the chances of the shell exploding as the "lottery of death." While he remembers many soldiers turning to liquor to quell their fears, Orelli always refused it when offered. In order to mentally prepare himself for an attack, overcome this fear and keep a clear head, he forced himself to purge his mind of all thoughts of home and family. Orelli was eventually wounded in the right leg, an injury that ended his military career and sent him home. Despite having vivid memories of the war itself, he recalled little of the events immediately following his injury. During a confrontation with a group of Austrian soldiers, he was wounded in the leg and the left ear, a wound that was only a few inches away from being fatal. After being taken to a nearby farmhouse, he spent the rest of the war recovering from infection in hospitals, of which he remembered little. Post-World War I

After the war, Orelli resumed his previous occupation as a mechanic. He had six children after the war, one son and five daughters born between 1920 and 1935, all of whom survived him. He also had nine grandchildren and eleven great-grandchildren at the time of his death. Although he was opposed to fascism, he found himself directing artillery in Gaeta during the Second World War, where he met his wife Cecilia. After the war, he returned to Rome and settled down with his family in Garbatella. He retired from his mechanic job in 1960 and his wife died in 1969. In his later years, he was active in telling tales of his experiences in the First World War, imploring those who listened to "not forget our sacrifice." In 2003, on the occasion of his 109th birthday, Italian President Carlo Azeglio Ciampi made Orelli a Grand Officer in the Order of Merit of the Italian Republic.

Towards the end of his life, he was known as "The Last Infantryman," which later became the title of the wartime memoirs that he published in 2004. At the time of his death, many sources incorrectly reported him as having been the last Italian World War I veteran. While this was incorrect, he was Italy's oldest survivor of the First World War, the last remaining trench infantryman and the last survivor from when Italy first entered the war in 1915. See also Veterans of the First World War who died in 2005