Neutrons are not emitted by most radionuclides that are used for medical or industrial purposes.
- Radiophamaceuticals and industrial radionuclides will set off gamma sensitive alarms. No legitimate reason to possess neutron source
- Detecting neutrons is usually unambiguous and indicates the presence of special nuclear materials.
VERY difficult to shield
- Shielding can even enhance the detection probability by increasing the thermalization and therefore the detection efficiency for thermal neutrons.
Neutron background is small and constant
- Neutron background rates throughout the world are remarkably constant.
- They vary from 0.007 n/cm2/sec at sea level to 0.014 n/cm2/sec near facilities like the Hanford Reservation in Washington State.
- These background rates are quite low compared to the expected count rates from small masses of metallic, oxide or weapons grade Pu.
Low false-alarm rate
- False positives are not acceptable when freight and people must be stopped and inspected when an alarm goes off.
How Are Neutrons Detected?
Neutrons have ~1 amu, but no electronic charge. Therefore, there is no columbic interactions, the primary method of energy loss for electrons and other charged particles. Epithermal and fast neutrons (>0.5 eV) may have long “path lengths”. Many centimeters of material may be penetrated.
Neutrons are usually “invisible” to standard detectors. When neutrons do undergo interaction, it is most often with a nucleus of the absorber material. The energy and direction of neutron is drastically changed.
If the neutron is captured, the capture reaction gives rise to reaction products in the form of secondary radiations and charged particles. These reaction products can be detected by conventional coulombic interactions within a detector through excitation or ionization.
Reaction 1 with 10B
10BF3 gas in tubes (0.5 to 1.0 atm)
Q = 2.792 MeV
It is used for detecting thermal neutrons, but 10B does have a large cross section for higher energy neutrons as well.
10B(n, α) reaction 10B + 1n -> 7Li + 4α
Thermal neutron x-section of 3840 barns but gas is toxic.
Reaction 2 with 3He
3He gas in pressurized tubes (1.0 to 20.0 atm)
Q = 0.763 MeV
Most commonly used for thermal neutron detection systems
3He (n, p) reaction 3He + 1n -> 3H + 1p
No solid state detectors are possible.
Thermal neutron x-section of 5339 Barns, but costly to cover large areas.
Reaction 3 with 6Li
An attractive alternative for slow neutron detection is the 6 Li reaction
Q = 4.78 MeV
6Li (n, α) reaction 6Li + 1n -> 3H + 4α
Reaction proceeds only to the ground state.
Thermal neutron x-section of 940 Barns, but lower x-section is offset by the higher Q.
There are more moles of 6Li in the glass fibers than there are moles of 3He in a pressurized gas tube. The atoms are also better spatially distributed to maximize sensitivity through optimization of the geometric efficiency.
What are the fibers' optical properties?
- Neutron glass scintillating fibers are sensitive to thermal neutrons.
- Enables large-area, small-size, solid-state, flexible, conformable and robust sensors.
- Exceeds best transmission performance reported in the open literature by a factor of 20 to 100.
- Neutron / gamma discrimination from 1250:1 to 8500:1
Tell me about the nuclear properties
- Fast neutrons are thermalized.
- Thermal neutron capture by 6Li
- Alpha particle and Tritium atom produced
- Triton excites Ce 3+ ion.
- Ce 3+ fluoresces and visible photons transmitted through fiber to PMT
The bulk composition of the high soda glass incorporates large weight fractions of both 6Li and Ce. How does the scintillating glass fiber technology work? The physics of the interaction begin with moderation of the neutrons. The 6Li atom has a large cross-section for thermal neutrons. Thermalization can occur anywhere from the neutron source, typically Pu atoms, to the detector. A good neutron moderator to maximize Thermalization before the neutron reaches the glass fiber usually surrounds the sensors.
Once a neutron is incident on the fibers, there is nearly 50% efficiency for its interaction within the glass. A 6Li atom absorbs the thermal neutron and the reaction products include a Tritium ion and an alpha particle. Although the range of alpha particles is small, there is a high probability that the alpha particle will interact with a Ce atom in the glass causing one of its electrons to be raised to an excited state. The de-excitation of the Ce atom’s electron to a ground state results in the emission of visible light, a fluorescence event.
The light is guided down the fiber until its interacts with one of the sensors photo-multiplier tubes. Scintillation can also result from gamma rays that produce energetic photoelectrons in the glass.
Neutron Sensitive Glass Fibers
Nucsafe uses a variety of advanced technologies in our products. One of our core technologies is a scintillating glass fiber for neutron detection. It represents a compelling alternative technology for thermal neutron detection versus 3He and BF3 tubes. Its key advantages over gas tubes are its sensitivity, ruggedness, flexible geometry, fast timing, and dynamic range. More details can be found in a primer on the 6Li neutron sensitive glass fiber technology.
The scintillating glass fibers work by incorporating 6Li and Ce3+ into the glass bulk composition. The 6Li has a high cross-section for thermal neutron capture. The capture reaction produces a tritium ion and an alpha particle and kinetic energy. The triton ion will likely interact with a Cerium ion through Coulombic interactions. This interaction results in the excitation of one of the Cerium atoms electrons. The resulting de-excitation of the electron produces a flash of light. This scintillation propagates through the glass fiber which acts as its own wave guide. The fibers are optically coupled to a photo-multiplier tube. At this point, the light is multiplied and converted to a electronic pulse that can be processed and counted.
Neutron Detector Technology – The Long and Short of It
Both 3He gas tube and PUMA 6Li scintillating glass fiber optic sensors can be incorporated into a multitude of detector geometries. Each detector technology has benefits and disadvantages. Nucsafe offers both types of neutron detectors in order to use the best technology for each application. In fact, Nucsafe can offer hybrid systems incorporating both types of detector technology in a single system to offer the best of both worlds.
3He gas tubes are the most commonly employed thermal neutron detector. They are a type of proportional detector, the most basic type of radiation detector. Each tube has an active length and diameter and are filled to a certain gas pressure. The length times the diameter yields a volume and the number of atmospheres of pressure defines how much gas is in that volume. The universal gas law can be used to compute the number of moles or atoms of gas that a particular 3He tube contains. Gas pressures range from 2 to 20 atmospheres and typical tubes are 3 to 10 atmospheres. The tubes are typically made of Al or Stainless Steel. The major benefit of 3He detectors is their insensitivity to gamma rays. The major disadvantages is their slow response, sensitivity to vibration and rigid geometry.
Thermal neutrons are captured by the 3He atoms that produce charged particles. The charged particles ionize other gas in the tube giving rise to ions and electrons that are accelerated toward their respective contacts and inducing a current in the preamplifier circuitry.
The 6Li scintillating glass fiber sensors are a solid state alternative to 3He gas tubes. The glass contains both the neutron sensitive atom 6Li and a scintillator of Ce. The fibers offer greater versatility to match the sample geometry and are flexible enabling wearable devices . These fibers are laid down in ribbons that can vary in length and number, ranging from 1 centimeter to 2 meters in length and 1cm to 16cm in width. Larger widths are made using multiple ribbons. The sensors can consist of a single fiber or tens of thousands of fibers, depending on the neutron flux to be detected. Each sensor is constructed of multiple layers of fiber interleaved with polyethylene, Teflon or other materials specific to the application. Because it is a glass, the distribution of the atoms is uniform throughout the glass fiber providing a uniform efficiency over the entire area of the detector. Another important advantage of the fiber technology is their fast scintillation time. By using high-speed electronics, these detectors have an improved response time. Ionizing radiation interacts with the scintillating fibers and produces light. This light is trapped in the fiber and goes to the fiber end. Here, conversion to an electrical signal takes place. This electrical signal is interpreted as either a neutron or gamma ray interaction, depending on its size.
The principal disadvantage of the glass fibers are their higher sensitivity to gamma rays. Using them in high gamma ray fields that are more than 100,000 times higher than the neutron flux does give rise to false neutron counts. Because the neutron count rate in the ambient environment is so low, even a few false counts can give rise to a count rate that is significant with respect to background.