## Determining the Intrinsic Neutron Detection Efficiency of a CdZnTe Detector Using Foil Activation
The
intrinsic neutron detection efficiency of a
CdZnTe (Cadmium Zinc Telluride) detector can be determined by using a technique known as
foil activation. This method involves irradiating a
neutron-sensitive foil with thermal neutrons and measuring the
induced radioactivity in the foil. The gamma radiation emitted during the decay of the activated foil can then be detected by the
CdZnTe detector. The process allows for the determination of the efficiency of the detector in capturing neutrons and converting them into measurable signals.
## 1. Understanding Foil Activation
Foil activation involves using materials (usually thin foils of neutron-capturing elements) that undergo nuclear reactions when exposed to neutrons. The most common process used in foil activation for neutron detection is the
neutron capture reaction, where a
neutron is absorbed by an atom, leading to the emission of
gamma rays or the creation of a
radioactive isotope.
For neutron detection using
CdZnTe detectors, materials like
Cd,
B, or
Li are often used as
activation foils, with
113Cd being one of the most commonly employed isotopes due to its high
neutron capture cross-section.
The specific reaction used in the activation process for
CdZnTe detectors is:
$$
^{113}Cd + n
ightarrow ^{114}Cd^*
ightarrow ^{114}Cd + gamma
$$
This reaction produces a
gamma-ray as the
114Cd nucleus de-excites. By detecting the
gamma rays emitted from the activated foil, the efficiency of the
CdZnTe detector in capturing neutrons can be evaluated.
## 2. Steps to Determine Neutron Detection Efficiency
The process of determining the
intrinsic neutron detection efficiency using foil activation involves the following steps:
## 2.1 Preparation of the Activation Foil
A thin foil of
neutron-sensitive material, such as
113Cd, is selected based on its high
neutron capture cross-section. The foil is placed in close proximity to the
CdZnTe detector for irradiation. The thickness and material of the foil are chosen such that the
neutron flux interacting with the foil is sufficient to induce measurable radioactivity.
## 2.2 Neutron Irradiation
The foil is exposed to a known
neutron flux in a neutron source, such as a
nuclear reactor or a
neutron generator. During this exposure, the neutrons interact with the foil, causing the activation reaction (e.g.,
113Cd (n, γ) 114Cd). The neutron flux and energy spectrum should be well-characterized to ensure accurate efficiency measurements.
## 2.3 Gamma Ray Detection
After the irradiation period, the
activated foil will begin to emit
gamma rays as the
114Cd isotope undergoes radioactive decay. The
CdZnTe detector is positioned to measure the
gamma rays emitted by the activated foil. The gamma spectrum recorded by the detector will include the
prompt gamma rays emitted during neutron capture, as well as any subsequent decay gamma rays.
## 2.4 Calculation of the Neutron Detection Efficiency
The intrinsic neutron detection efficiency of the
CdZnTe detector is determined by comparing the measured gamma-ray intensity with the
known neutron flux. Several factors need to be taken into account during the calculation:
*
Gamma-ray Yield: The yield of gamma rays emitted from the foil during the activation process is proportional to the number of neutrons captured. This can be calculated based on the
neutron capture cross-section of the foil material and the total number of neutrons absorbed.
*
Detector Response: The gamma-ray response of the
CdZnTe detector is characterized by the
energy resolution and
detection efficiency for gamma rays. This response can be determined through calibration with known gamma-ray sources.
*
Background Subtraction: Background gamma radiation from other sources in the environment or from the neutron source itself must be subtracted from the measured gamma spectrum to isolate the signal from the activated foil.
## 2.5 Efficiency Formula
The intrinsic neutron detection efficiency
η can be expressed as:
$$
η = frac{N_{gamma}}{N_{ ext{neutrons}}}
$$
Where:
* $N_{gamma}$ is the number of
gamma rays detected by the
CdZnTe detector (after background subtraction).
* $N_{ ext{neutrons}}$ is the number of
neutrons interacting with the foil, which is related to the
neutron flux and the
foil's neutron capture cross-section.
This calculation provides the intrinsic efficiency of the
CdZnTe detector in detecting thermal neutrons, factoring in the
detector's response to gamma rays.
## 3. Advantages of Foil Activation for Neutron Detection Efficiency
*
Direct Measurement: Foil activation provides a direct way to measure the
neutron detection efficiency by using a well-understood activation process and gamma-ray emission.
*
Calibration: It can be used as a calibration method for
CdZnTe detectors, especially for
space-based missions or
field deployments, where precise neutron fluxes are difficult to measure directly.
*
Non-Destructive: The foil activation process is non-destructive to the
CdZnTe detector, making it a convenient method for evaluating detector performance without damage.
*
Versatility: The technique can be applied to different materials, and by varying the material of the activation foil (such as
113Cd,
B, or
Li), it can be used for a wide range of neutron detection scenarios.
## 4. Challenges and Considerations
While foil activation is a powerful technique for measuring neutron detection efficiency, several factors must be carefully controlled:
*
Neutron Spectrum: The efficiency measured is sensitive to the
energy spectrum of the neutrons. For accurate efficiency calculation, the neutron spectrum should match the operational conditions of the detector.
*
Detector Geometry: The position and geometry of both the
CdZnTe detector and the
activation foil need to be optimized for accurate efficiency measurements.
*
Background Radiation: It is important to account for any
background gamma radiation from external sources or the irradiation process, which can influence the gamma-ray spectrum and efficiency calculation.
## 5. Conclusion
Foil activation is an effective and widely used method for determining the
intrinsic neutron detection efficiency of
CdZnTe detectors. By measuring the
gamma rays emitted from
neutron capture reactions in the foil, the efficiency of the
CdZnTe detector in detecting thermal neutrons can be quantified. This method provides valuable data for
calibration and
performance assessment in a variety of neutron detection applications, including
nuclear security,
environmental monitoring, and
space-based missions.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/blog/how-is-the-intrinsic-neutron-detection-efficiency-of-a-cdznte-detector-determined-using-foil-activation.html
