FAQ - Moderators, Neutron moderation, what it does, how to do it, why do it?
Posted: Wed Mar 13, 2013 10:59 am
Much contained in this FAQ is already in print here, but scattered. I was stunned to find this subject not in a FAQ here. I hope Carl Willis will add to this important effort, as his knowledge here far exceeds my own.
Moderation for detecting and counting neutrons:
Neutrons produced by the fusor are all considered "Fast neutrons". All neutron sources tend to produce fast neutrons. All of the best detectors for neutrons rely on "thermal neutrons" to be detected by some secondary process.
For detection of fusor produced and any man made source of neutrons, (fast), it is incumbent on the would-be neutron metrologist to slow these neutrons down to thermal velocities allowing detectors sensitive to these "slow neutrons" to count them.
The act of slowing down fast neutrons to create slow or "thermal" neutrons is called "moderation". The assembly needed to do this is called a "moderator".
Moderators can be made of a number of materials, but moderators made of hydrogen rich materials work best and take up the least amount of space. Moderation or "slowing" of neutrons occurs when neutrons effectively "hit" a proton within the hydrogen of the moderator causing the proton to recoil, much like a billiard ball. Of course, just like the billiard ball, the proton is sent recoiling off as some of the cue ball's, (neutron's), momentum is transferred or stolen from it. Thus, the proton flies off while the neutron is slowed down. Note, that both particles, like the billiard balls, are "scattered" off at angles not necessarily along the path of the original cue ball (neutron).
If the moderating material is thick enough, many such collisions of the scattered neutrons are possible and more slowing down is seen to occur as well as more wild scattering within the material.
There is a point where a beam of fast neutrons entering the moderator, "hydrogenous material", are slowed and scattered to such a high degree that it, the moderator, may appear as a neutron source of isotropically emitted "slow neutrons". These neutrons are said to be "thermalized".
Thermal neutrons are those neutrons that have a velocity equal to that of the atoms and molecules of their surrounding environment. They are in thermal equilibrium with all that is around them. (At the same effective temperature). Most all neutron counter detection schemes respond to thermal neutrons. Thus, the need for moderators and neutron moderation.
The detector is generally placed within the moderator such that any fast neutron beam that enters the moderator will thermalize to such a degree that the detector will be bathed in a sea of thermalized, easily counted, neutrons from all directions.
Moderation for neutron activation:
Neutron activation of common materials is a separate study, whereby, various materials are bombarded by neutrons in an attempt to "activate" or make them radioactive. This effort usually demands a source of thermal neutrons. Therefore, moderation is also valuable in this area of study as well.
The moderator - materials and sizing:
The preferred moderator materials most encountered in amateur use are, in order of use....
1. Low Density Polyethylene "LDPE"
2. High Density Polyethylene "HDPE"
3. Water
4. Paraffin wax
The polyethylenes can be obtained in large cities at a commercial or retail plastics dealer in both cylinder and thick sheet forms. Pre-cut pieces can be assembled or built up into a moderator.
Water is a great, zero cost, moderator, but requires a tank and must be maintained, (evaporation, airborne dirt and dust, etc) Tanks are subject to leakage and breakage.
Water allows for extreme fine-tuning of detection and activation by moving the detector or material to be activated around the tank for more or less moderation.
Paraffin is easily obtained at many grocery stores in block or plate form. However, paraffin is flammable and poses a fire hazard when assembled in large amounts, should a fire break out in the lab. Prior to modern plastics, Paraffin was the number one moderator used in instrumentation and in small laboratories.
The above list is not complete at all, but has served the amateur and even the professional communities well in the past and at present.
Thickness of the moderator:
The moderator should, ideally, be a sphere for detection and measuring purposes, but this is not always practical. In general, a detector or material to be activated will need about 2-3 inches of the above moderating material surrounding it on all sides.
More on moderating for activation:
Less thickness in a moderator will slow neutrons to "epi-thermal" velocities where a lot of activation materials have what are termed "capture resonances" to neutrons of energies just above thermal energies. These resonances can present monstrously huge cross sections for "neutron capture" within the material to be activated. This means that in some activation scenarios, a broad band of resonances and large capture cross sections might best be utilized for activation where the neutrons are at less than thermal energies.
If a moderator is made so large that the neutrons are slowed to velocities well below that of the surrounding molecules, they are termed "cold neutrons". For detection purposes, there is not much advantage for obtaining cold neutrons. However for activation studies cold neutrons, below thermal energy, increase the capture cross section and improve activation on a more or less linear scale based on the classic 1/V law. (neutron physics) There is a limit, however, as the neutron flux (neutrons per sq. cm.) is reduced in this effort due to increasing moderator volume and a trade off point is reached for any given incident flux of fast neutrons outside the moderator.
There is much more to this story to which it is hoped others will contribute in replies. However, this is the "quick rinse" promised on moderation and moderators.
I attach my simple "neutron oven" image I have used to activate Silver, Indium and Rhodium. (HDPE)
Richard Hull
Moderation for detecting and counting neutrons:
Neutrons produced by the fusor are all considered "Fast neutrons". All neutron sources tend to produce fast neutrons. All of the best detectors for neutrons rely on "thermal neutrons" to be detected by some secondary process.
For detection of fusor produced and any man made source of neutrons, (fast), it is incumbent on the would-be neutron metrologist to slow these neutrons down to thermal velocities allowing detectors sensitive to these "slow neutrons" to count them.
The act of slowing down fast neutrons to create slow or "thermal" neutrons is called "moderation". The assembly needed to do this is called a "moderator".
Moderators can be made of a number of materials, but moderators made of hydrogen rich materials work best and take up the least amount of space. Moderation or "slowing" of neutrons occurs when neutrons effectively "hit" a proton within the hydrogen of the moderator causing the proton to recoil, much like a billiard ball. Of course, just like the billiard ball, the proton is sent recoiling off as some of the cue ball's, (neutron's), momentum is transferred or stolen from it. Thus, the proton flies off while the neutron is slowed down. Note, that both particles, like the billiard balls, are "scattered" off at angles not necessarily along the path of the original cue ball (neutron).
If the moderating material is thick enough, many such collisions of the scattered neutrons are possible and more slowing down is seen to occur as well as more wild scattering within the material.
There is a point where a beam of fast neutrons entering the moderator, "hydrogenous material", are slowed and scattered to such a high degree that it, the moderator, may appear as a neutron source of isotropically emitted "slow neutrons". These neutrons are said to be "thermalized".
Thermal neutrons are those neutrons that have a velocity equal to that of the atoms and molecules of their surrounding environment. They are in thermal equilibrium with all that is around them. (At the same effective temperature). Most all neutron counter detection schemes respond to thermal neutrons. Thus, the need for moderators and neutron moderation.
The detector is generally placed within the moderator such that any fast neutron beam that enters the moderator will thermalize to such a degree that the detector will be bathed in a sea of thermalized, easily counted, neutrons from all directions.
Moderation for neutron activation:
Neutron activation of common materials is a separate study, whereby, various materials are bombarded by neutrons in an attempt to "activate" or make them radioactive. This effort usually demands a source of thermal neutrons. Therefore, moderation is also valuable in this area of study as well.
The moderator - materials and sizing:
The preferred moderator materials most encountered in amateur use are, in order of use....
1. Low Density Polyethylene "LDPE"
2. High Density Polyethylene "HDPE"
3. Water
4. Paraffin wax
The polyethylenes can be obtained in large cities at a commercial or retail plastics dealer in both cylinder and thick sheet forms. Pre-cut pieces can be assembled or built up into a moderator.
Water is a great, zero cost, moderator, but requires a tank and must be maintained, (evaporation, airborne dirt and dust, etc) Tanks are subject to leakage and breakage.
Water allows for extreme fine-tuning of detection and activation by moving the detector or material to be activated around the tank for more or less moderation.
Paraffin is easily obtained at many grocery stores in block or plate form. However, paraffin is flammable and poses a fire hazard when assembled in large amounts, should a fire break out in the lab. Prior to modern plastics, Paraffin was the number one moderator used in instrumentation and in small laboratories.
The above list is not complete at all, but has served the amateur and even the professional communities well in the past and at present.
Thickness of the moderator:
The moderator should, ideally, be a sphere for detection and measuring purposes, but this is not always practical. In general, a detector or material to be activated will need about 2-3 inches of the above moderating material surrounding it on all sides.
More on moderating for activation:
Less thickness in a moderator will slow neutrons to "epi-thermal" velocities where a lot of activation materials have what are termed "capture resonances" to neutrons of energies just above thermal energies. These resonances can present monstrously huge cross sections for "neutron capture" within the material to be activated. This means that in some activation scenarios, a broad band of resonances and large capture cross sections might best be utilized for activation where the neutrons are at less than thermal energies.
If a moderator is made so large that the neutrons are slowed to velocities well below that of the surrounding molecules, they are termed "cold neutrons". For detection purposes, there is not much advantage for obtaining cold neutrons. However for activation studies cold neutrons, below thermal energy, increase the capture cross section and improve activation on a more or less linear scale based on the classic 1/V law. (neutron physics) There is a limit, however, as the neutron flux (neutrons per sq. cm.) is reduced in this effort due to increasing moderator volume and a trade off point is reached for any given incident flux of fast neutrons outside the moderator.
There is much more to this story to which it is hoped others will contribute in replies. However, this is the "quick rinse" promised on moderation and moderators.
I attach my simple "neutron oven" image I have used to activate Silver, Indium and Rhodium. (HDPE)
Richard Hull