SemeaTech gamma sensors utilize advanced Cesium Iodide (CsI) scintillator technology combined with Silicon PIN photodiodes (Hamamatsu S5106 for 3cc gamma sensor). These components make the sensors highly durable and reliable, offering a significantly longer lifespan compared to Geiger-Müller (GM) tube sensors. As a result, they serve as an excellent replacement for standard Geiger counters, particularly for use as alarm detectors. Key benefits of SemeaTech gamma sensors include:
a.Durability: The CsI scintillator is exceptionally robust, capable of withstanding various environmental conditions while maintaining consistent performance, even at temperatures as low as -20°C.
b.Ease of Use: Designed for simplicity, the sensors operate in a plug-and-play manner. By connecting the sensor to an oscilloscope, users can immediately detect pulses, such as with a radiation source like kBq Cs-137, without the need for additional components.
SemeaTech gamma sensors are not inherently designed for underwater use but can detect gamma radiation signals when properly encapsulated.
No, gamma sensors cannot reliably detect or measure energy below 50 keV. At room temperature, the noise generated by the photodiode overwhelms signals lower than 50 keV, making it impossible to isolate or measure them accurately.
The lifespan of a gamma sensor is influenced by factors such as aging and corrosion. However, the sensor can operate continuously as long as its enclosure and internal circuitry remain intact and free from corrosion, ensuring that its sensitivity is preserved.
It’s important to note that the lifespan mentioned in the datasheet corresponds to the warranty period, which does not necessarily reflect the sensor’s true operational lifetime. In the absence of material damage, gamma sensors can continue functioning reliably over time without their performance being significantly affected by age.
SemeaTech gamma sensors are designed to function optimally for a lifespan of 5 years under normal operating conditions. Its warranty period of 15 months begins from the date the sensor is shipped from the manufacturing facility, ensuring its performance during this timeframe. While the specified lifespan and warranty provide a guideline, the gamma sensor may have a prolonged functional lifetime due to its construction with a cesium-iodide crystal. This material does not degrade over time, suggesting that the sensor’s longevity could exceed its design expectations when used in suitable conditions.
No, there is no risk of SemeaTech gamma sensors becoming radioactive. This is due to the selection of construction materials and components:
a.Cesium Iodide Crystal: This stable material does not emit radiation and cannot become radioactive, even after exposure to radiation.
b.PIN Diode and Electronics: These components are inherently non-radioactive and do not undergo any interactions that would cause them to emit radiation.
Additionally, SemeaTech gamma sensors do not contain any radioactive substances, nor are they made from materials susceptible to activation (becoming radioactive after exposure to radiation sources).
SemeaTech utilizes a compact radiation source of Cesium-137 with an activity level of 0.25 μCi for testing gamma sensors. During testing, the gamma sensor is positioned 10 mm away from the radiation source, regardless of its orientation.
Cesium-137 is selected as the radiation source for SemeaTech gamma sensors due to its medium energy level, making it suitable for industrial and environmental applications. It is widely used in environmental monitoring and serves as a calibration standard for radiation detectors.
1 µCi corresponds to 37,000 decays per second. Therefore, 0.25 µCi
corresponds to approximately 9,000 decays per second. About 85% of
cesium-137 (Cs-137) decays are accompanied by gamma ray (photon)
emissions. As a result:
• For 1 µCi of Cs-137, there are approximately 37,000 decays per
second, of which about 37,000 × 0.85 ≈ 31,450 decays per second
are accompanied by gamma ray emissions.
• For 0.25 µCi of Cs-137, approximately 9,000 × 0.85 ≈ 7,650
decays per second are accompanied by gamma ray emissions.
Other radiation sources have distinct characteristics:
Cobalt-60: Emits high-energy gamma rays, ideal
for deep penetration tasks such as sterilizing medical equipment
and food irradiation. It is commonly used for calibrating gamma
detection instruments.
Americium-241: Produces low-energy gamma rays, making it suitable for close-range applications like ionization-type smoke detectors and thickness gauging in manufacturing. Its long half-life ensures durability and reliability for continuous operation.
Radiation intensity and energy are distinct concepts:
• Each radioactive isotope emits gamma rays with a specific energy
level, which is a fixed characteristic of that isotope. For
example:
o Americium-241 (Am-241) emits gamma rays at one energy level,
o Cesium-137 (Cs-137) at another,
o Cobalt-60 (Co-60) at yet another.
• Energy is typically measured in kilo-electron volts (keV) and
determines the penetrating power of the radiation. Higher-energy
gamma rays penetrate materials more easily.
• Radiation intensity (or activity) is measured in units like
microcuries (µCi) or micro sieverts per hour (µSv/h). It reflects
how many gamma photons are emitted per second by the source.
Important Note: Always attend training sessions provided by certified professionals before handling any radiation sources. These materials can pose significant risks to human health if not managed properly. Proper education and adherence to safety protocols are essential to prevent accidents and ensure compliance with regulations.
We recommend the following precautions:
• Sealed Storage During Use: Always store radioactive materials in
a sealed plastic bag during use. This minimizes the risk of
contamination by preventing direct contact with surfaces or the
spread of particles.
• Limiting Exposure: Handle radioactive materials minimally and
keep them sealed whenever possible. This reduces the risk of
radiation exposure to personnel and the environment.
• Post-Use Storage: After testing, store radioactive materials in
a metal container. Metal containers provide effective shielding,
limiting radiation emissions and ensuring safe storage.
The gamma sensor features a CsI crystal, elongated in shape and positioned laterally at one end. Its large surface areas exhibit consistently high sensitivity, ensuring reliable performance across these orientations.
The wires are soldered onto the printed circuit board (PCB), creating a robust electrical connection. Epoxy resin is then applied around the wires and soldered joints, providing protection against environmental elements such as moisture, dust, and physical damage.
Temperature has minimal impact on the detection of gamma photons due to their high energy and the physical nature of radiation detection, which is inherently temperature insensitive. Changes in ambient temperature within the sensor's operating range (-20°C to 60°C) do not significantly affect the Cesium Iodide (CsI) crystal's detection efficiency of gamma photons or the magnitude of the pulse amplitude from photoelectric conversion.
However, temperature does influence the performance of the photodiode. At temperatures above 40°C, the amplitude of Silicon PIN photodiode noise increases significantly, and at 50°C, the noise can drown out pulses generated by low-energy gamma rays. Conversely, when transitioning from a high-temperature environment to a low-temperature one, the noise level decreases as the sensor reaches thermal equilibrium, and the opposite occurs when moving from low to high temperatures. Without proper temperature compensation, the pulse count rate decreases when moving from high to low temperatures, and increases from low to high temperatures.
For the Silicon PIN photodiode (Hamamatsu S5106) used in 3cc gamma sensor. As the temperature increases, these variations become more pronounced. For instance, one gamma sensor may register 100 pulses to indicate 30 µREM/hr, while another may register 150 pulses for the same dose rate. On the other hand, the photodiode (Hamamatsu S5106) is very sensitive to ambient temperature especially when the temperature increases to 40 degree C and above. This is why gamma sensors must be calibrated after being installed in gamma monitors, and the gamma monitor must have thermistors to compensate for the ambient temperature variations.
It’s worth noting that radiation sources are not required for temperature calibration. Instead, calibration involves verifying the sensor's performance across its temperature range by analyzing how noise levels behave under different conditions. Additionally, a pulse amplitude screener is commonly used to separate gamma radiation pulses from noise. The screener's threshold is typically set to the noise height of the photodiode, but as temperature rises, the increased noise amplitude necessitates raising the screening level. This adjustment can block some low-energy gamma photon pulses, potentially reducing the pulse count rate in the user’s application system (not the gamma sensor itself).
Since the relationship between noise and temperature lacks fixed quantitative metrics, each gamma sensor must be calibrated in actual applications, as the performance of individual photodiodes can vary.
At ambient temperatures reaching 35°C, thermal noise can interfere with and overlap radiation signal outputs. To counteract this, a higher-intensity radiation source is required to produce a more distinguishable output. Once the pulses are detected (with a lower pulse count in this case), a compensatory software algorithm must be implemented to maintain the linearity of the sensor outputs. Americium-241 is too weak and doesn’t work for it. Using a higher-energy Cesium-137 should be the solution.
To calibrate the sensor, begin by establishing its baseline noise level in the absence of any Cs-137 source. Once the baseline is recorded, introduce the Cs-137 and measure the sensor’s response to determine the change in output relative to the background level.
In SemeaTech production, gamma sensors are not precisely calibrated because calibration is not critical for the sensor itself and is more relevant to the application system in which the sensors will be used. A reference radiation source is used to verify the sensor’s performance, ensuring the sensor's noise level and gamma ray response meet design specifications.
When formal calibration is not feasible, alternative test methods
- often referred to as "bump tests"—can be used to verify the
basic functionality of gamma sensors. These tests involve exposing
the sensor to low-dose, stable radiation sources to confirm its
ability to detect gamma radiation and generate corresponding
signal pulses. While these methods do not replace full
calibration, they are effective for functional checks and quality
assurance. Here are the recommended test methods:
• Smoke Detector Ionization Chambers - it contains small amounts
of Americium-241 (Am-241), a weak gamma-emitting isotope.
• Commonly used in bump tests by repurposing one or more
ionization chambers from commercial smoke detectors.
• Lantern Mantles - Some older mantles contain trace amounts of
thorium, which emits low levels of gamma radiation. They can be
served as accessible check sources for verifying sensor
response.
These sources emit relatively weak radiation. To achieve a
detectable signal, it may be necessary to stack multiple items or
position them close to the sensor.
Gamma sensors occasionally experience higher-than-expected background counts due to electronic noise-random fluctuations in the circuit that generate meaningless small pulses. These noise pulses can interfere with the detection of actual signals, particularly when no radiation source is present.
To address this, the discrimination threshold comes into play. This threshold is the voltage level at which the comparator distinguishes valid signal pulses from noise. By increasing the reference voltage on the comparator’s negative pin, the threshold can be raised, ensuring that noise pulses are filtered out while meaningful signals are retained. It's crucial to find a balance: the threshold must block all noise pulses without filtering out valid signals.
In practice, threshold adjustments should be dynamic and tailored to the specific characteristics of each gamma sensor. Noise levels and sensitivity vary between sensors, so calibrating the reference voltage accordingly is essential. The threshold isn't a fixed value - it should be set based on the magnitude of electronic noise generated by the sensor. For sensors with higher noise levels, a higher threshold may be necessary to avoid detecting noise as signals.
To determine the appropriate threshold, an oscilloscope can be used to measure and visualize the noise pulses, providing clarity on pulse heights. When no radiation source is present, users should observe the natural environment’s noise pulses, including environmental radiation or electromagnetic interference. These observations will guide the reference voltage settings, effectively filtering out noise while maintaining signal integrity.
Sensor-to-sensor variation can occur due to several factors, including the position of the sensor relative to the radiation source. A slight shift in the sensor’s position or angle can affect how much radiation it detects, resulting in variations in CPS (counts per second) readings. Sensors that are closer to or differently aligned with the source may record differing levels of radiation.
Another key factor is pulse overlap in environments with high radiation intensity. Radiation pulses that occur close together in time may overlap, creating challenges for the sensor in accurately counting individual pulses. If overlapping is significant, the sensor might misinterpret multiple pulses as a single pulse, leading to inaccurate CPS measurements. For example, when two pulses occur in quick succession but are counted as one, the CPS reading will appear lower than it should.
Pulse width, or the duration of a pulse, also plays a role in sensor-to-sensor variation. Slight differences in the electronic components of each sensor can result in variations in pulse width. Wider pulses increase the likelihood of overlap, as additional pulses are more likely to occur during the detection window of the first pulse. This increases the risk of saturation, reducing the sensor's ability to count pulses accurately and leading to variations in CPS readings.
In contrast, low radiation fields have pulses spaced further apart, minimizing the risk of overlap. This allows sensors to detect and count pulses with greater accuracy, as saturation and overlap are less likely to occur.
Operating conditions can significantly influence the performance
of gamma sensors:
a. Sensitivity to Vibrations: The mini gamma sensor is highly
sensitive to mechanical vibrations, which can cause undesired
noise or distortions in readings due to the microphonic effect.
This effect occurs when vibrations generate electric noise within
the sensor, compromising its performance. To mitigate this issue,
protective measures should be implemented in system applications.
For instance, using a rubber socket to secure the sensor and
additional components to stabilize the cable can help reduce
vibrations and enhance sensor reliability.
b. Electromagnetic Interference (EMI): The mini gamma sensor is
also susceptible to EMI from wireless communication devices, which
can disrupt its operation and result in unreliable or inaccurate
readings. There is no solution for suppressing the microphonic
effect. In contrast, the 3cc gamma sensor’s robust structural
design makes it resistant to such interference.
For these reasons, we strongly recommend using 3cc gamma sensors in such operating conditions. Mini gamma sensors, on the other hand, are not suitable for use near wireless instruments or in environments with significant vibrations.
While we do not currently have formal performance data specific to ESD and EMC test standards, SemeaTech utilizes professional radiation and EMI test equipment to evaluate sensor robustness during development. In the design phase of the 3cc gamma sensor, we conducted functional assessments by exposing the sensor to typical sources of electromagnetic interference, including mobile phones and walkie-talkies operated within a 0.5-meter range. These tests confirmed that the 3cc gamma sensor is not susceptible to microphonic effects under such conditions. In contrast, the mini gamma sensor showed higher sensitivity to microphonic interference in the same environment.
The 3cc Gamma sensor is equipped with two layers of EMI shielding, whereas the Mini Gamma sensor has only a single layer. To reduce susceptibility to EMI, we recommend enhancing the shielding on the Mini Gamma sensor by i) Adding a second shielding layer with a gap or insulating material between layers. ii) Ensuring the shield fully encloses the sensor with no gaps. iii) Shielding both ground and power wires as much as possible.
Gamma sensors primarily convert incident gamma rays into electrical impulses, focusing on detecting and registering the energy of incoming radiation. They do not incorporate energy compensation mechanisms to adjust or correct for variations in the intensity or energy levels of the radiation. This means fluctuations in the energy or intensity of incoming gamma rays are not factored into their measurements.
T90, or the response time, refers to how fast a gamma sensor is response to a radiation source. The term "T90" originates from quality control practices and denotes the time required for sensor responses to stabilize and become repeatable within a defined timeframe. For the mini gamma sensor, the T90 is about 10 seconds. In comparison, the 3cc gamma sensor exhibits a T90 of about 1 second. This rapid response can be observed on an oscilloscope as the signal curve rises and flattens into a horizontal line, signifying a stable reading. The period from the initial response to the horizontal line is defined as the T90 or response time.
The baseline of a gamma sensor is its normal output under background radiation conditions. It reflects the ambient gamma levels from natural sources and serves as a reference point for detecting abnormal increases. A stable baseline ensures accurate readings and proper sensor function.
The gamma sensors are highly sensitive to microphonics, which refer to vibrations or mechanical shocks that induce noise or fluctuations in the sensor ‘s output signal. For example, striking the sensor during operation causes the output rail to oscillate between saturation and ground (GND) multiple times, which indicates that mechanical shock leads to significant signal disturbances, or "vibrations," that can disrupt system performance.
To mitigate these effects, gamma sensors should be mechanically
mounted in a way that minimizes the transmission of vibrations.
Here’s the recommended solution list:
1. Using Rubber Socket: Rubber Socket can absorb mechanical shocks
and reduce direct impact of vibrations on the sensor.
2. Fixing the Cable: Secure sensor’s cable to prevent additional
movement or vibrations that could exacerbate the issue.
3. Optimizing Placement: Place the sensor in a location that is
less prone to mechanical shocks or vibrations during system
operation.
4. Isolating the Sensor: Put the sensor on a vibration-damping
platform or use shock-absobing materials in the housing to isolate
it from external mechanical disturbances.
These operations help enhance sensor’s stability and reduce the impact of microphonics on the performance.
The sievert (Sv) is a measure of radiation dose that accounts for both the energy of the gamma radiation and the number of interactions (counts) per kilogram of tissue. It reflects the biological effect or damage equivalent to humans. In contrast, the curie (Ci) measures the activity of a radioactive source - specifically, the number of disintegrations per second - regardless of the energy of the emitted radiation. So, Ci is purely about how many particles are emitted, not how damaging they are.
Yes, changing isotopes will affect the Sv measurement because different isotopes emit gamma rays at different energy levels. While the count rate contributes significantly to the Sv value - especially for isotopes with similar energies - the total dose will not be identical across isotopes due to energy differences.
Sources are usually specified in activity units like gigabecquerels (GBq) or millicuries (mCi) because these reflect the number of disintegrations per second, which is a property of the source itself. Sv or gray (Gy) are dose units that depend on how the radiation interacts with matter, which varies based on energy, distance, and shielding.
In our case, we’re focusing on the number of pulses captured by the oscilloscope, which corresponds to the count rate. These counts contribute to the Sv calculation but don’t represent the full dose picture, since energy must also be considered. While counts are a major factor, energy differences between isotopes must be accounted for when interpreting dose.
