Neutrón - Wikipedia
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The smaller pieces are called "secondary" cosmic rays, and they in turn hit other air molecules resulting in more secondary cosmic rays. The process continues and is termed an "atmospheric cascade".
If the primary cosmic ray that started the cascade has energy over MeV, some of its secondary byproducts including neutrons will reach ground level where they can be detected by neutron monitors.
Neutron detection - Wikipedia
Measurement strategy[ edit ] Since they were invented by Prof. Simpson in there have been various types of neutron monitors. All neutron monitors however employ the same measurement strategy that exploits the dramatic difference in the way high and low energy neutrons interact with different nuclei.
There is almost no interaction between neutrons and electrons. High energy neutrons interact rarely but when they do they are able to disrupt nuclei, particularly heavy nuclei, producing many low energy neutrons in the process. Low energy neutrons have a much higher probability of interacting with nuclei, but these interactions are typically elastic like billiard ball collisions that transfer energy but do not change the structure of the nucleus.
The exceptions to this are a few specific nuclei most notably 10B and 3He that quickly absorb extremely low energy neutrons, then disintegrate releasing very energetic charged particles. With this behavior of neutron interactions in mind, Professor Simpson ingeniously selected the four main components of a neutron monitor: An outer shell of proton-rich material — paraffin in the early neutron monitors, polyethylene in the more modern ones. Low energy neutrons cannot penetrate this material, but are not absorbed by it.
Thus environmental, non-cosmic ray induced neutrons are kept out of the monitor and low energy neutrons generated in the lead are kept in. This material is largely transparent to the cosmic ray induced cascade neutrons. The producer is leadand by weight it is the major component of a neutron monitor.
Fast neutrons that get through the reflector interact with the lead to produce, on average about 10 much lower energy neutrons. This both amplifies the cosmic signal and produces neutrons that cannot easily escape the reflector. The moderator, also a proton rich material like the reflector, slows down the neutrons now confined within the reflector, which makes them more likely to be detected.
This is the heart of a neutron monitor. After very slow neutrons are generated by the reflector, producer, moderator, and so forth, they encounter a nucleus in the proportional counter and cause it to disintegrate. This nuclear reaction produces energetic charged particles that ionize gas in the proportional counter, producing an electrical signal.
The pressure in the accelerating region, however, has to be much lower, as the mean free path of electrons must be longer to prevent formation of a discharge between the high voltage electrodes. The ion beam can thus be focused to a small point at the target.
The accelerators typically require power supplies of - kV. The neutron production rate can also be controlled. Neutron tubes have several components including an ion source, ion optic elements, and a beam target; all of these are enclosed within a vacuum-tight enclosure. The neutron tube is, in turn, enclosed in a metal housing, the accelerator head, which is filled with a dielectric medium to insulate the high voltage elements of the tube from the operating area.
The accelerator and ion source high voltages are provided by external power supplies. The control console allows the operator to adjust the operating parameters of the neutron tube.
The power supplies and control equipment are normally located within 10—30 feet of the accelerator head in laboratory instruments, but may be several kilometers away in well logging instruments. In comparison with their predecessors, sealed neutron tubes do not require vacuum pumps and gas sources for operation. They are therefore more mobile and compact, while also durable and reliable.
For example, sealed neutron tubes have replaced radioactive neutron initiatorsin supplying a pulse of neutrons to the imploding core of modern nuclear weapons. Examples of neutron tube ideas date as far back as the s, pre-nuclear weapons era, by German scientists filing a German patent Marchpatentand obtaining a United States Patent JulyUSP 2, ; examples of present state of the art are given by developments such as the Neutristor,  a mostly solid state device, resembling a computer chip, invented at Sandia National Laboratories in Albuquerque NM.
Ion source A good ion source should provide a strong ion beam without consuming much of the gas. For hydrogen isotopes, production of atomic ions is favored over molecular ions, as atomic ions have higher neutron yield on collision. The ions generated in the ion source are then extracted by an electric field into the accelerator region, and accelerated towards the target.
The gas consumption is chiefly caused by the pressure difference between the ion generating and ion accelerating spaces that has to be maintained. Cold cathode Penning [ edit ] The Penning source is a low gas pressure, cold cathode ion source which utilizes crossed electric and magnetic fields. The ion source anode is at a positive potential, either dc or pulsed, with respect to the source cathode.
The ion source voltage is normally between 2 and 7 kilovolts. A magnetic field, oriented parallel to the source axis, is produced by a permanent magnet. A plasma is formed along the axis of the anode which traps electrons which, in turn, ionize gas in the source.
The ions are extracted through the exit cathode. This disadvantage is however compensated for by the other advantages of the system. One of the cathodes is a cup made of soft ironenclosing most of the discharge space. The bottom of the cup has a hole through which most of the generated ions are ejected by the magnetic field into the acceleration space.
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The soft iron shields the acceleration space from the magnetic field, to prevent a breakdown. The schematic indicates that the exit cathode is at ground potential and the target is at high negative potential. This is the case in many sealed tube neutron generators. However, in cases when it is desired to deliver the maximum flux to a sample, it is desirable to operate the neutron tube with the target grounded and the source floating at high positive potential.
The accelerator voltage is normally between 80 and kilovolts. The accelerating electrode has the shape of a long hollow cylinder. The ion beam has a slightly diverging angle about 0. The electrode shape and distance from target can be chosen so the entire target surface is bombarded with ions. The ions pass through the accelerating electrode and strike the target.