How Are Thermal Neutrons Formed and Why Are They Important in Physics?
In 1920, a neutron was first theorized by Ernest Rutherford and then discovered by an English Physicist named James Chadwick in 1932. Therefore, Chadwick was awarded the Nobel prize in physics for the discovery of the same in the year 1935.
A Thermal Neutron is a neutron that is in thermal/warm balance with the surrounding medium.
These sorts of neutrons have a neutron speed of 2200 m/s at surrounding temperature states of 293.6 Kelvin relating to energy of 0.0253 eV.
On this page, you will find all the information on three different parameters of a thermal neutron, viz: thermal neutron energy, thermal fission, and kinetic energy of thermal neutrons.
Thermal Neutron Definition
A thermal neutron or a free neutron (one that isn't bound inside a nuclear core) has an average energy of movement (kinetic energy) in comparison to the average energy of the particles of the encompassing materials.
Moderately slow and of low energy, thermal neutrons show properties, like enormous cross segments in parting, that make them attractive in certain chain-reaction applications.
Moreover, the long de Broglie frequencies of thermal neutrons make them significant for specific uses of neutron optics.
Thermal neutrons are created by hindering more enthusiastic neutrons in a substance called an arbitrator after they have been launched out from nuclear cores during atomic reactions like fission. This process is also known as thermal fission.
Thermal Neutron Formation
Neutron is a subatomic particle arranged inside the core of the iota. It is electrically neutral (for example chargeless particles) and has a mass marginally higher than that of the proton.
Neutrons, along with protons, are called nucleons. At the point when the neutron stays inside the core, it remains exceptionally steady and can be ousted by atomic change as it were. In any case, when a neutron stays outside the core, it turns out to be profoundly flimsy/unstable and goes through radioactive decay into a proton, an electron, and an antineutrino with the half-existence of around 10 minutes. Such a neutron is called the free neutron.
This free neutron has a few applications, the most eminent one is the inception of the nuclear fission reaction. In parting, the heavier core parts into at least two lighter cores when the previous one is barraged by the high-speed neutrons.
In light of the energy of the free neutron, it very well may be characterized into a few gatherings - each gathering comprises reach-in neutron energy. Cold neutrons, Thermal neutrons, Cadmium neutron, Slow neutron, Fast neutron, and so forth are not many gatherings of free neutrons having various scopes of energy.
Here, we are focusing on a thermal neutron, so let’s discuss its properties in detail:
Properties of a Thermal Neutron
A thermal neutron stays in warm harmony with the surrounding particles at Normal Temperature and Pressure (NTP). This shows the average kinetic energy of thermal neutrons is the same as that of any gas atom at 20°C.
A free thermal neutron has energy in the request for 0.025 eV (minor deviation conceivable).
The velocity of a thermal neutron is approximately 2.2 km/s.
All such nuclear reactors that work essentially dependent on the thermal neutrons are called Thermal Reactors.
Thermal neutron offers a high parting cross-area (around 583 barns) towards Uranium-235 (it is the most well-known fuel isotope for atomic reactors).
Prompt neutrons delivered in nuclear fission are fast neutrons. Accordingly, a moderator is needed in thermal reactors to hinder the prompt neutron speed with the goal that such neutrons can initiate further fission to support a chain reaction.
In this day and age, thermal neutrons have broad applications in nuclear power plants.
Now, let us understand the concept of thermal energy:
Thermal Energy of Neutron
Quantitatively, the thermal energy per particle is approximately 0.025 electron volt. It is a measure of energy that relates to a neutron speed of around 2,000 meters each second and a neutron frequency of around 2 × 10-10 metres (or around two angstroms).
Since the frequency of thermal neutrons relates to the normal spacings between atoms in glasslike/crystalline solids, light emissions neutrons are ideal for researching the design of precious stones, especially for finding places of hydrogen atoms, which are not very much situated by X-ray diffraction methods.
(Thermal Neutrons Use)
Thermal neutrons are needed for initiating atomic parting in normally happening Uranium-235 and in misleadingly delivered Plutonium-239 and Uranium-233.
Kinetic Energy of Thermal Neutron Formula
According to Maxwellian distribution theory, the kinetic energy of thermal neutron distribution is Maxwellian, which is given by the following kinetic energy of thermal neutron formula:
E = 12mn v2
The number/quantity of neutrons of energy "E" per unit energy span "N (E)," and the quantity of neutrons "v" per unit velocity interval, can be expressed as;
dN0/dE = N (E) = (2πN0)/(πkBT)3/2 \[\sqrt{E}\]e-E/kBT
Here, kB is Boltzmann's constant, whose value is 8.617333262 x 10-5 eV/K or 1.38 x 10-23 J/K.
There’s a device that detects the thermal neutron presence, and that is a thermal neutron detector, let’s understand it:
Thermal Neutron Detector
A detection/Identification of neutrons is very specific since the neutrons are electrically neutral particles, hence they are chiefly dependent upon strong nuclear forces yet not on electric forces. Thus neutrons are not straightforwardly ionizing and they have generally to be changed over into charged particles before they can be distinguished.
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By and large, every kind of neutron detector should be furnished with a converter (to change neutron radiation over to basic recognizable radiation) and one of the regular radiation detectors are a scintillation detector, gaseous detector, semiconductor detector.
Now, let’s have a look at some FAQs on our main topic, Thermal Neutron:
FAQs on Thermal Neutron: Properties, Formation & Applications
1. What is a thermal neutron as per the CBSE Class 12 syllabus?
A thermal neutron is a free neutron (one not bound within an atomic nucleus) that has been slowed down sufficiently to be in thermal equilibrium with the surrounding medium. Its average kinetic energy corresponds to the ambient temperature. The term 'thermal' is used because its energy distribution follows the Maxwellian distribution, similar to particles in thermal motion.
2. What is the typical kinetic energy and speed of a thermal neutron?
At standard room temperature (approximately 20°C or 293 K), a thermal neutron possesses an average kinetic energy of about 0.025 eV (electron-volts). This energy level corresponds to a speed of approximately 2.2 km/s.
3. How is the kinetic energy of a thermal neutron related to temperature?
The average kinetic energy (E) of a thermal neutron is directly proportional to the absolute temperature (T) of the medium it is in. This fundamental relationship is described by the equation E = kT, where 'k' is the Boltzmann constant (approximately 1.38 x 10⁻²³ J/K).
4. What is the importance of thermal neutrons in a nuclear reactor?
Thermal neutrons are critically important for sustaining a nuclear chain reaction in most reactors, particularly those using Uranium-235. The key reasons for their importance are:
- High Fission Probability: Fissile nuclei like Uranium-235 have a much higher probability (a larger 'nuclear cross-section') of capturing a slow-moving thermal neutron compared to a high-energy fast neutron.
- Sustained Chain Reaction: This efficient capture leads to nuclear fission, which releases a significant amount of energy and more neutrons, thus sustaining the chain reaction required for power generation.
5. What is the main difference between a thermal neutron and a fast neutron?
The primary difference between a thermal neutron and a fast neutron is their kinetic energy, which in turn affects their role in nuclear reactions.
- Thermal Neutrons possess very low kinetic energy (~0.025 eV), move at slower speeds (~2.2 km/s), and are highly effective at inducing fission in nuclei like U-235.
- Fast Neutrons are emitted directly during fission and have very high kinetic energy (~2 MeV or 2,000,000 eV). They are less likely to be captured by U-235 but can cause fission in nuclei like U-238.
6. Why are thermal neutrons more effective at causing the fission of Uranium-235 than fast neutrons?
The increased effectiveness of thermal neutrons is due to the concept of nuclear cross-section, which is the effective target area a nucleus presents for a particular reaction. The fission cross-section of Uranium-235 is inversely proportional to the neutron's velocity. Therefore, a slow-moving thermal neutron spends more time in the vicinity of the nucleus, drastically increasing the probability of it being 'captured'. This capture makes the resulting U-236 nucleus highly unstable, causing it to undergo fission almost instantly.
7. How are fast neutrons converted into thermal neutrons inside a nuclear reactor?
Fast neutrons are slowed down to become thermal neutrons through a process called moderation. This is achieved by making them undergo multiple elastic collisions with the nuclei of a moderator material. During each collision, the fast neutron transfers a fraction of its kinetic energy to the moderator's nucleus. Common moderator materials have light nuclei to maximise energy transfer per collision. Examples include:
- Heavy water (D₂O)
- Graphite (Carbon)
- Ordinary water (H₂O)
8. Can thermal neutrons cause fission in all heavy elements like uranium and plutonium?
No, thermal fission is not efficient for all heavy nuclei. It works best for nuclei that have an odd number of neutrons, which are referred to as 'fissile' materials. Examples include Uranium-235, Uranium-233, and Plutonium-239. Nuclei with an even number of neutrons, like Uranium-238, are 'fissionable' but not fissile, meaning they require high-energy fast neutrons (with energy > 1 MeV) to undergo fission.
