#4 FAQ - Glossary of terms, Definitions, Fusor Specific

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Richard Hull
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#4 FAQ - Glossary of terms, Definitions, Fusor Specific

Post by Richard Hull » Thu Sep 06, 2012 11:27 pm

NOTE TO USERS: This glossary is designed for a quick rinse explanation of terms and designed with a total newcomer in mind and not a physics major. We have trimmed and simplified the definitions and terms to suit only fusor specific question from newbies confused by the terms. To delve deeper, should that be necessary, you must "hit th'books"

*****************************************************************

General description -

This is to be an expanding group of common "fusioneer speak" terms that are often referred to by the old boys and others doing fusion that might not be clear to a newbie.

It is expected that all newbs will be self-driven to learn, to some useful depth, not only the terms, but also what they mean in the grand scheme of things.

This list will be organized alphabetically. It will bloom and then balloon, for sure, to a point where it is too big for one FAQ. It will then be broken into FAQs with the same title and a suffix like (A-E), etc.

I want all to contribute in replies either with corrections, suggestions of new terms to define or submit their own word with definition. If it passes muster it will be addressed and added.

Definitions should be sufficient to basic understanding, but not tutorial in nature. (short and sweet)

Suggest only those terms that you or others have had issues with in the beginning.

Definition credits by initials. First initial - suggester-poster, following intials are modifications or additions by others.

CB - Chris Bradley
DB - Dennis Brown
RH - Richard Hull

*********************************** A *****************************************

Atmosphere (atm) - The normal air pressure found at sea level. 14.7 PSI or 760mm of mercury. See full definition under (STP) RH

*********************************** B *****************************************

'Bake-out' - (vacuum term) When the whole chamber, or parts of it (e.g. a fore-line trap), are heated up so that any adsorbed gases on their surfaces are displaced and sucked out. Note* bake out of a fusor is almost never done nor is it really demanded at any time. Foreline traps, very rarely used, do demand bakeout but often contain their own internal heater. CB RH

'Base pressure' - The minimum pressure you can achieve with your system. This is determined and limited by the type and condition of your pumps, vacuum lines and the degree to which your system is sealed against the outside atmospheric pressure. CB RH


************************************ C ***************************************

CF fitting - (see conflat fitting) RH

Christmas tree - (Also called "the stack"). The internal jet assembly that is found inside of a diffusion pump. So called because of its general outline and appearance. All Diff pumps have them and any would-be buyer should be sure that any used diff pump they intend to purchase has this vital component for it is easily removed and, in some models, is not internally retained and will fall out if the pump is inverted. RH

Conflat fitting - (abreviation "CF") A special vacuum hardware fitting used to seal or connect professional vacuum system components. It utilizes a machined knife edge on each mating fitting's face designed to dig into and deform a copper gasket to seal the system against any leaks from the outside. The gasket is deformed as bolts around the periphery of the mated fittings are tightened in a specified order. Considered to be among the best seals in the vacuum business. Expensive in larger sizes RH

Cross section - a complex concept that must be studied by the student of fusion.
Particles present an imaginary "target size" (cross section) to similar particles based on their kineitc energy. In general, the more kinetic energy, the larger the target cross section and the more likely a fusion will result from their union via the quantum tunneling process. A range of kinetic energies means a range of probabilities for fusion. Graphs are experimentally generated and are presented as a representation of relative fusion probabilities relating a colliding particle's kinetic energy plotted against an increasinging cross sectional scale. For deuterium-deuterium collisions, energies as low as 15kev, (kilo-electron volts) can start a fusion process. As the energy is increased to 30kev, the cross section swells and the probability of colliisional fusion and, thus quantum tunneling increases. RH

*********************************** D ***************************************

D-D fusion (reaction) - Deuterium-Deuterium fusion. This is the reaction seen in all of our fusors. When deuterons collide with deuterons and fuse, as in our fusors, a specific set of reactions occur at the time of fusion. Deuterons can fuse in a manner that results in three possible reactions, only two of which are the norm. These two reactions occur with about equal or 50:50 probability when averaged out over many fusions.

1. D + D = 3He (rare helium3) + n (neutron)
2. D + D = T (Tritium - radioactive hydrogen with two neutrons) + p (proton)

All of the four particles are energetic (in motion) and have the following energies

1. D + D = 3He (0.82 mev) + n (2.45mev)
2. D + D = T (1.1mev) + p (3.02 mev)

There is a third ultra-rare reaction that may occur in about 1 in 100,000 fusions

3. D + D = 4He (normal helium) + gamma ray

The gamma ray is very energetic at nearly 24 mev! The reaction is so rare that it is virtually never discussed when speaking of D -D fusion.

As we are normally concerned with counting neutrons to determine how good our fusion is going, 2X10e5 neutrons per second means that we have actually fused 4X10e5 times as the second reaction is hidden, but occurs with equal probability and had to occur 2X105 times by itself in order for us to count the same number of neutrons. RH CB

Demo fusor - (demo system) A simple bell jar or metal fusor that operates and looks like a real fusor except that there is no deuterium gas in the system and it is not doing fusion. For consideration to enter the plasma club, you must have a central grid focus of the plasma. Simple plasma in a tube will not do. A demo fusor is usually a recommended first pass for a student or often seen in smaller science fairs. It primary advange is its low cost, ease of operation at voltage much lower than needed to do fusion. In general, a full metal demo system can go right into being a real fusor by the addition of a deuterium gas system and a higher voltage power supply. Bell jar demo fusors cannot easily or safely be used to do any significant fusion. The demo fusor gives the exact look and feel of a real fusor. You develop operational skills on a demo fusor and learn all about plasmas by the doing. RH

Diffusion pump - (abreviation - "diff pump") An oil filled pump that is connected to a forepump or mechanical pump to further reduce the pressure in a fusor to sub micron levels. (often to 10e-5 or 10e-6 torr). This pump is now considered a "must have" item as it warrants that a fusor chamber is far below a micron so that when deuterium gas is admitted, to micron levels, the atmosphere in the fusor will be mostly, (95%+), deuterium. It utilizes hot oil vapor created in its internal "boiler" directed through numerous jet nozzles at supersonic velocity to entrain remnant gas molecules in "molecular flow". This action forces these molecules out of the diff pump's exhaust and into the forepump's inlet line for removal from the vacuum system. RH

Deuterium - This is an isotope of hydrogen, (proton), with a neutron added to its nucleus. Chemically, it is still, simply, hydrogen, but has been given its special name of deuterium. Like hydrogen, it is a flammable/explosive gas when mixed with oxygen. Deuterium is the fusion fuel used in the amateur fusor. No fusor has yet fused without it. It remains the only nuclear fusion fuel available to the amateur experimenter that is unregulated and relatively inexpensive. It is a "must have" gas inorder to do amateur fusion. Normally obtained from better welding supply houses (special order) or from specialty scientific gas suppliers (who the welding shops buy from) RH

Deuteron - A naked deuterium nucleus, (ion), that has lost its single orbital electron. Fusion occurs more readily if only deuterons collide with each other at specific energies. Obtaining an environment of only deuterons, all with the ideal specific energy, is nearly impossible, but the fusor manages to have some small fraction of its particulates meet this criterion. RH, CB

*********************************** E ****************************************

Electron - A negatively charged particle that is found in all elemental atoms and is found in a "cloud" about the nucleus. Electrons are easily stripped from what is called the outer electronic shell of any atom by using large potential gradients. This is the main method used in the fusor for creating deuterons, (naked or bare deuterium nuclei), Deuterium only has one electron encircling its nucleus. when this is stripped away, the bare nucleus is called a deuteron. These are what are classically needed to do efficient fusion in any fusion reactor. RH

Electron volt - A unit of energy commonly applied to nuclear particles in motion, (kinetic energy). It is not an electrical potential, but a kinetic energy based on a particles mass and velocity. Classically, 1 ev is the energy acquired by a particle of unit charge accelerated through a potential gradient of one volt. (boring).... Suffice it to say that it is a tiny unit of energy and is equal to 1.6 X10e-19 watt. (16 Quintillionths of a watt). Particle kinetic energies are normally expressed in kilo or mega electron volts. The neutron released in deuterium fusion, (in our fusors), has an average energy of 2.45 mega-electron volts, (2.45mev). For all its millions of electron volts, we would need over 30 trillion 2.45 mev neutrons to represent a single watt of energy! Note** When we create a deuteron near the fusor's anode and it accelerates towards the inner grid at 20 kilovolt applied, it has the possibility of obtaining an ultimate kinetic energy of 20kev once it gets there. RH, CB

*********************************** F ***************************************

'Fore-line trap' - an additional, optional component on the vacuum-inlet side of the forepump (before the chamber/turbo-pump and/or diffusion pump) that contains a material which is highly porous with a very high surface area in which substances will become adsorbed. This helps pumping, and also helps minimise back-streaming gases from the forepump, (gases and oil vapor that may work their way back towards the vacuum chamber). The foreline trap will be equipped with a heater element so that it is easily heated up and the adsorbed substances driven off and "sucked" out whilst the chamber valve is closed. Note* Rarely used in fusor work. CB RH

Forepump - (see mechanical pump) RH

Fusion energy - A term applied to particles, (in our case, deuterons), that posess a certain minimum kinetic energy where quantum tunneling will allow fusion to take palce. The energy needed occurs over a wide range based on what is termed a particle's collisional cross section. (see cross section). All fusion requires input or "seed" energy applied from outside the fusion fuel environment. RH

Fusion - The forced union of two nuclei via a quantum tunneling process. All fusion requires input or "seed" energy. Many fusions of light nuclei are exothermic (create more energy than required to cause their fusion). This is the draw of nuclear fusion...To create energy for man's use. RH

Fusor - An all metal fusion reaction chamber. Most often, in amateur hands, a simple two electrode system in which a spherical metal shell is the anode and an inner grid, the cathode. High potential is applied between the shell and the grid to a deuterium gas environment of about 10 microns pressure within the fusor. A glow discharge is initialized. Deuterium atoms are ionized to become deuterons. These are then accelerated to do "collisional" fusion via quantum tunneling as well as by a number of other possible mechanisms and conditions favorable to fusion. Amateur fusors using D-D fusion can easily achieve 100,000 fusions per second and have recorded rates as high as 6,000,000 fusions per second. RH

*********************************** G *****************************************

Gamma ray - An electromagnetic particle-like entity of extreme energy and not part of the normal fusion reactions in the fusor, but for those delving into neutron activation and the study of isotopes, gamma ray spectrometry is used to establish successful isotope creation. Gamma rays are the result of energetic nuclear processes from within the nucleus of an atom. Gamma energies typically range from ~50kev to many mev. RH

Gas handling system - A complete setup attached to the fusor that delivers operator controlled amounts of deuterium gas into the fusor. It, traditionally, is made up of a tank of high pressure deuterium gas, connected to a pressure reducing gas regulator, followed by a small gas line to a micro metering valve or other user controlled "leak" and then out of there through a smaller gas line and into the fusor via a vacuum rated gas fitting. RH

Grid - (Central or inner grid) Normally, a hollow geodesic sphere made up of small wires either welded or brazed together. It is the central, core electrode in a metallic, spherical fusor and is the cathode or negative electrode used to accelerate deuterons towards the center of the fusor. The normal diameter of this grid rarely exceeds 2-inches. It is normally fashioned out of Stainless steel wire in demo fusors and of a more refractory metal such as Tungsten, Molybdenum, or tantalum in formal, successful, fusing fusors. RH

********************************** I ****************************************

Ignition - A point in a hypothetically imagined environment that allows continuous containment of fusion fuel that is at fusion energy, where fusion begins and the resultant excess fusion based energy self-feeds the reaction to the point where no more outside input or "seed" energy is required. Once ignition has occured in a fusion reactor, the only thing needed is a continuous input of fusion fuel, much as in a gasoline engine. The fusor is not, nor will it ever be, capable of ignition. Ignition is the ultimate goal in a thermo-nuclear reactor of the magneitcally confined, gas plasma type reactor. RH


********************************** K *****************************************

Kelvin - (abreviaton K) Another of those scientific energy expressions usually confused with temperature, but definitely linked to it via the motion of molecules. For our purposes 1 ev equals ~ 11,600 kelvins of kinetic energy. Zero Kelvins is a point on the temperature scale where all molecular motion and thus all heat disappears. Sometimes referred to as absolute zero. For thermal purposes the kelvin unit at STP is equal to centigrade thermal units, though the scales zero points are different. i.e. a nuclear particle with 2 mev of Kinetic energy is said to be at 11,600 X 2 million or 23.2 billion kelvins. If we had these particles in a density that would equal standard sea level pressure conditions the particle group would be at approx. 22 billion degrees C! However, the single particle is incapable of beng at that temperature in the sense that we know as thermal heat, but it does satisfy all the condtions of being at the energy equal to a particle at atmospheric pressure of the same energy at 22 billion degrees C. Thus kelvins are often deliberatly misused by fusion physics in this manner to imply that a fusion reaction needs to have the particles at billions of degrees. All fusioneers must laugh at this kind of pronouncement as it is all publicity hype. The near equivalence of unit Kelvins and degrees C is strictly related to a molecular or particle density at full atmospheric pressure. RH

KF fitting - KF = Klein Flansch (small flange) , A vacuum hardware fitting designed to seal and join various vacuum system components. Also called a "Quick Fitting" (QF). This is an easily dismantled fitting used where a part of the vacuum system needs to be changed often. It finds its major application in technical vacuum work down to 1 micron pressures. (Note* KF fittings can go to higher vacuum in experienced hands, but in general use, they are never used in vacuums in excess of 10e-6 torr). The KF fitting relies on a thumbscrew clamp to seal two fittings that are flat faced but have their rear surfaces beveled. Tighening the clamp forces the flat faces together with a special low vapor pressure polymer gasket in between to make the seal. Far less expensive than the rather costly Conflat or "CF" fittings. RH, CB, DB


*********************************** M ****************************************

Mechanical pump - (also called a forepump) A two stage, oil filled vacuum pump designed to achive a technical vacuum near one micron is pressure. It is a must have item to even begin to think about fusor construction. These pumps are of the "indirect drive" and "direct drive" types. Indirect drive pumps use a separate motor and pump connected by a V belt running over pullys on each. A direct drive pump has the motor and pump shafts directly connected with no belt or pulleys. RH

Micron - very roughly a millionth of an atmosphere. More on this later, but it is the common pressure term used in "technical vacuums" by technical and scientific people when talking amongst themselves. The fusor operates in the micron range if internal pressure and all simple pressure metering that the amateur will most likely obtain are calibrated in microns. Mechanical or "forepumps" used in all fusor setups are typically capable of pumping, by themselves, into the 20 to 5 micron pressure range. The micron is not a term currently used in peer reviewed published works. RH

********************************* N *****************************************

Neutron - A neutral or uncharged particle contained within the nucleus of all atoms except simple hydrogen. Its mass is only a tiny bit more than that of a proton. The neutron is the only one of four particles created in deuterium fusion that exits the fusor! Thus, its measure, taken on the outside of the fusor, is the only proof that fusion is occuring. Detecting and counting neutrons is a rather daunting task for the amateur. In the end, it is the object of the game here! No neutrons = no fusion. RH

********************************** O ****************************************

'Out-gassing' - the process of sublimation of materials, and/or when adsorbed gases get released from surfaces. 'Out-gassing' is one possible cause of 'virtual-leaks'. These substances get released into the vacuum chamber, contaminate the gases in it, and increase the chamber pressure. Out-gassing tends to reduce over time as the chamber is held under vacuum and pumped down regularly. Heating up the chamber surfaces (e.g. during operation) increases out-gassing rates, which means out-gassing rates will reduce quicker than when the chamber is cold. CB


*********************************** P ***************************************

Proton - a positively charge particle in the nucleus of all atoms. During deuterium fusion within our fusors, a proton is one of the four particles resulting from the fusion process. As it is charged, it can't penetrate the shell of the fusor and mostly winds up as simple hydrogen gas once it combines with free electrons within the fusor. RH

********************************* Q *****************************************

QF fitting - (see KF fitting) RH

Quantum tunneling - Far too complicated to expalin in detail, but here goes.
Two deuterons are positively charged. As they approach each other at high speed their charges tend to electrostatically repel each other and wish to refuse to allow contact. However, in the mysterious, spooky and probablistic world of quantum theory, there is a chance of the deuterons tunneling through this repulsive force to collide and fuse using the ultra close range, attractive force in the nucleus called the "Strong Force". Once tunneled, the nuclear strong force takes over and babada-da-bing, babada-da-boom....FUSION. All fusion in a fusor is via the spooky, yet good offices of quantum tunneling. RH



********************************** S **************************************

sccm - 'Standard Cubic Centimetres per Minute'; a rate of the mass flow of a gas given independently of its pressure, but measured as if it were at STP. A term most often used in calibrated "mass flow controllers" often found within gas handling systems CB

Scientific notation - A method of writing and easily computing with incredibly huge numbers of units or an incredibly small fraction of a unit. It is a "must understand" concept as it is used extensively by engineers, scientists and amateur fusioneers. Consult a good math book for more data. i.e. 10,000 is written 1X10e4 or just 10e4 with the one as understood. 23,400 is written 2.34X10e4. .00026 is written 2.6X10e-4 to multiply 23,400 X .00026 we multiply 2.34 X 2.6 = 6.08 X10e0....10e0 is 1 so the answer is 6.08. With mental rounding of simple single digit prefix units and mental addition and subtraction of exponents (the "e" numbers), we can do complex big number math in our heads to simple order of magnitude accuracy! Scientific notation is a powerful tool. RH

Scientific vacuum - A deep or high vacuum environment that is in the 10e-6 torr range or better. Such vacuums are generally only required in scientific work. These vacuums always require a mechanical or forepump plus an additional pump in line together. This additional pump is normally a diffusion pump or a turbo-pump. RH

Shell - The metal sphere that is the grounded anode or positive electrode in a fusor. In all metal systems, it is the spherical body or fusor chamber. In Bell jar fusor "demo" systems, it usually takes the form of a larger, >5", wire grid structure surrounding the inner grid. RH

"Stack" - See Christmas tree

STP - (Standard Temperature and Pressure) Also called an "atmosphere".
This is defined as a sea level pressure of 760 millimeters of mercury or 760 "torr" at 0 degrees centigrade. All vacuum work and pressures are measured against 1 atmosphere - STP and are, thus, small fractions of it. RH

Swagelock fitting - A trade name for a common type of gas line fitting often used to couple gas handling system components. RH

"Sweet Spot"- a term particular to, and defined within these forums by successful fusioneers. A sweet spot is an operational point obtained during the running and adjustment of a fusor where the voltage, current and fusor's internal pressure are such that continuous operation is possible with only minor operator tweeking to keep it at the current operating level. A sweet spot need not be the absolute optimal operating point for any given fusor where maximum fusion is obtained, though it certainly could be, provided it is stable. It is just a point of smooth operation that eases the operator's tension which often results from constant adjustment details. RH

*************************************** T *******************************************

Technical vacuum - A vacuum or reduced pressure environment that is in the single micron range. Technical vacuums are usually obtained by the use of a simple mechanical pump, alone. RH

TIER - Total Isotropically Emitted Radiation. The total count of particulate radiation emitted over an idealized sphere surrounding a source of radiation, usually expressed in particles emitted per second. Used with the fusor related to neutron radiation emission values. Most often quoted neutron specification when discussing neutron radiation from a fusor. RH

Torr - A unit of pressure equal to one millimeter of mercury a "standard atmosphere" is 760mm or 760 torr. 1 millitorr = 10e-3 torr = 1 micron. In common practice, all pressures below 1 micron are considered "scientific vacuums" and are expressed as a scientific notation in torr units. i.e., .05 microns = 5X10e-5 torr.
The torr is another effectively abandoned unit and is not used in peer reviewed papers. Professionals talking among themesleves, privately, commonly use torr units, however. RH

It is fine to not use the micron at all, but as 100% of the operational range of all fusors are in the single digit micron range, the use of the micron saves a bit of scientific notational jargon. It is in bad taste, however, to use tiny fractional units of a micron. Revert to notational torr units. RH

Turbo-pump - (Turbo-molecular pump). This is a rotary vaned pump that spins at very high rpm with hundreds of tiny blades in many stacked rotors which successively hit remnant molecules of gas within the pump and force them to leave via its exhaust port which is connected to the inlet port of a mechanical pump. Turbo-pumps are very expensive compared to a diffusion pump and demand a matching electronic controller to power them. These pumps are considered "dry pumps" as they do not use oil vapor in their operation. RH

************************************** V **********************************************

Vacuum - A conditon of reduced pressure compared to normal sea level atmospheric pressure. There is no perfect vacuum! There is always pressure! The pressure is created by thermal collisons of gas molecules. All vacuums, no matter how poor or how complete, have a measurable, reportable pressure. Fusors work in a vacuum of reduced pressure where the atmospheric gases are effectively replaced by deuterium. a fusor would have a desired operating pressure of about 1/100,000ths of an atmosphere. (10e-5 atm). RH

Vacuum leak - An real opening or fault in what attempts to be a closed and sealed vacuum system. This opening is often very small. This leak allows outside atmospheric gas to enter a system under vacuum. RH CB

VCR fitting - A type of gas line fitting used by professional and scientific users to assemble gas handling systems entering a very low pressure vacuum chamber. These fittings are very expensive and the amateru often opts for the more common an less expensive "Swagelock" type gas line fittings. RH

'Virtual Leak' - (see- "out-gassing") A source of gases from within the vacuum envelope that is difficult to pump out (e.g. a partially trapped pocket of gas, or substances that slowly sublimate over time). [cf 'Real leak' where there is a breach in the vacuum envelope that, unintentionally, allows atmospheric gases in.] CB


************************************** X *******************************************

X- ray - An electromagnetic particle-like packet of energy emittied from within an atom's electronic shell. X-rays are part of the fusor's output, but not due to any fusion process. Instead, they are produced when electrons impact the shell of the fusor. In a metallic shelled fusor, x-rays can become a radiation issue around 30 kilovolts applied. As the voltage climbs over 40kev, thin lead shielding is recommended or remote operation of the device. In all fusors, the x-radiation is the number one radiation concern, far overpowering the neutron radiation in relative danger. Fusors operated over 60 kilovolts will demand very special shileding arrangements to afford protection to operators and bystanders. RH
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: FAQ - Glossary of terms, Definitions

Post by Dennis P Brown » Sat Sep 08, 2012 1:16 am

A minor point but KF flanges are good to 10^-6 torr (used them to reach that level all the time with great success. Have reach 10^-7 torr with five KF fittings and a good turbo for a very large bell jar.)

What of Steradian (Sr)? It is the SI unit of solid angle where 4Pi Sr is the solid angle that creates a 'sphere' around a measured source. Smaller solid angles are important for neutron flux measurements in papers and used for most equipment scales. Really just the name and def. so people can look it up on the wiki and are aware of the unit.

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Richard Hull
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Re: FAQ - Glossary of terms, Definitions

Post by Richard Hull » Mon Sep 10, 2012 7:51 pm

Dennis,

I did put in a proviso' on the KF fittings.

Steradians are not commonly used here in the verbage. If encountered in outside reading, the readers can look it up. Common, oft repeated stuff that noobs read here that might confuse, please.

Chris,

I used all of your suggestions either verbatum or with some additions I felt obliged to add.

Credit initials after definitions.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Benjamin Abbatiello
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Re: FAQ - Glossary of terms, Definitions, Fusor Specific

Post by Benjamin Abbatiello » Fri Mar 21, 2014 3:07 pm

Just a quick question about the rare reaction...
How is the reaction with a gamma ray being emitted exceedingly rare if it happens in about every 1 on 100,000 fusions?
As you said in this topic, Richard: viewtopic.php?f=11&t=9075&p=61961&hilit ... ond#p61961
"A 6" diameter metal fusor can do and has done 2 million fusions per second at 43,000 volts"
This means that there is an 0.00001% chance of this happening in one fusion, and at 2m fusions per second, how is that seen as rare?
I am probably making a very obvious mistake, but I was just curious.
It is a great act of cleverness to be able to conceal one's being clever.
-Francois de La Rochefoucauld

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Re: FAQ - Glossary of terms, Definitions, Fusor Specific

Post by Tom McCarthy » Fri Mar 21, 2014 5:11 pm

Benjamin,
The 3rd reaction, or rare as you call it, is described as "exceedingly rare" because as you noted it has a 0.00001% chance of happening. Note that I said chance, this does not guarantee that it will happen once out of every 100,000 fusions.
A second point is that the Oxford Dictionary explains rare to mean "(of an object)not found in large numbers and so of interest or value" and "(Of an event, situation, or condition) not occurring very often". Myself, I would view the terms "not occurring very often" and "not found in large numbers" to be relative to the situation. For example, cystic fibrosis is a rare genetic disorder of which around 1 in 29 people are a carrier and if we take Great Britain as an example, 1 in 2,500 people actually have the disease. So, if we assume that 1 out of 100,000 collisions produce a gamma ray, then that is 20 out of every 2 million, which I would view as very rare.

Also, in future, please try to refrain from posting on FAQs and open up a discussion thread as well. Most posts on FAQs are liable to deletion.

Tom

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