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Detector specifications
All Medusa spectrometers are built around our proprietary embedded processing system, implementing fast FPGA-based (*) digital signal processing. The Medusa Detector Operating System (mDOS) performs high-speed full-waveform sampling of the scintillator crystal output, connecting to either classic Photomultiplier tubes or modern Silicon Photomultiplier (SiPM) readout systems. Full-waveform signal processing implies for zero-deadtime operation - an important feature needed in high-countrate applications, with counting speeds up to 250kcps.
The spectra acquired are fully linearised using a built-in "Poisson-preserving" version of the well-known Medusa Full Spectrum Analysis method, allowing for the seamless addition of spectra from any number of records of one or even multiple individual detectors. Spectra are stored internally in 4096 channels. The channel resolution can be increased to 16384 channels upon request.
Our detectors normally are equipped with very robust CsI crystals. Other materials, like NaI, CeBr or BGO are available on request.
1.1. Nomenclature and typical use
Medusa detectors are named as follows: "MS-xxx YYY"; where MS = "Medusa System"; "xxx" denotes the (approximate) detector volume in ml and YYY the crystal type (+application).
| System | Variants | Application type |
|---|---|---|
| MS-50 | CsI or BGO | - Sensor for 3rd party borehole tools; - Sensor for Medusa RhoC density meter |
| MS-100 | CsI or CeBr | - gSMS (Stationary gamma Soil Moisture Meter) - (Underwater) pipeline inspection systems - Chemical plant inspection systems |
| MS-350 | CsI or CeBr | Geophysical and agricultural surveying - Handheld system for walking surveys - Droneborne surveys |
| MS-700 | CsI or CeBr | Geophysical and agricultural surveying - Handheld system for walking surveys - Driving surveys - Droneborne surveys |
| MS-1000 | CsI | Geophysical and agricultural surveying - Driving surveys - Droneborne surveys |
| MS-1200 | CsI | Underwater sediment mapping for geophysical and geochemical (pollution) studies |
| MS-2000 | CsI | Geophysical and agricultural surveying - Driving surveys |
| MS-4000 | CsI / NaI | Geophysical surveying - Driving surveys - Airborne surveys |
1.2. System comparison sheet
| Available Crystal types | |
|---|---|
| CsI | - Systems: All - Density: 4.5 kg/l - Energy resolution: typically better than 8.5% on ¹³⁷Cs (661 keV) - Robust, virtually unbreakable |
| CeBr | - Systems: MS-100; MS-350; MS-700 - Density 5.1 kg/l - Energy resolution: typically better than 3.9% on ¹³⁷Cs (661 keV) - Suited for applications needing peak analysis / high resolution / anthropogenic material detection (nuclear waste). |
| NaI | - Systems: MS-4000 - Density 3.7 kg/l - Energy resolution: typically better than 7.5% on ¹³⁷Cs (661 keV) - Low-cost but quite shock and temperature sensitive. - Not recommended for drone/walking/vehicle-borne use because of its fragility. |
| BGO | - Systems: MS-50 - Density: 7.2 kg/l - Energy resolution: typically better than 11% on ¹³⁷Cs (661 keV) - Suited for applications where sensor volume is restricted |
| Crystal electronics | |
| MS-50 | 1 inch PMT |
| MS-100 | 4 pcs Silicon PMs |
| MS-350; MS-700; MS-1000; MS-1200 | 6 pcs Silicon Photomultiplier (SiPM); CeBr versions contain a 3-inch PMT |
| MS-2000; MS-4000 | 3.5-inch or 4-inch PMT |
| Spectrometer electronics | |
| Channels | 4k (16k on request). |
| Nonlinearity | <0.2% (diff); <0.1 (integral). |
| Dynamic range | 2 Mcps (theoretical), 250 kcps (measured on a CsI crystal, ***). |
| Dead-time | zero (**). |
| Baseline restoration, pulse shaping, pile-up rejection | Digital (full-waveform sampling). |
| Spectrum sample rate | max 10 Hz to continuous. |
| Stabilisation | The system stabilises on natural peaks (40K, 238U and 232Th). FSA-based Poisson statistics are preserved. |
| Timing | Internal real-time clock |
| Embedded CPU (MS-100 up) | |
| Processor system | Toradex Verdin (NXP i.MX 8M Mini Cortex-A53 quad core) |
| Onboard storage | 16GB. Optional 64 GB |
| Real-time clock | PCF8523 |
| Embedded GPS | - uBlox ZED-F9P - RTK enabled, with a base station - Signals: BeiDou, Galileo, GLONASS, GPS / QZSS - Update rate: 1 Hz - Dynamic positioning precision: 7 mm + 1ppm - Static position precision: 4mm +0.5ppm |
| Embedded internet link | 4G e-SIM-based uplink (upon request) |
| Embedded pressure/temp/hum | - Bosch BME280 - Temperature range: -40 ... 85 °C - Pressure range: 300 ... 1100 hPa - Humidity 0 ... 100% - Update rate:1 Hz / 5 Hz (on request) |
| I/O | WIFI, Ethernet, RS232 upon request. |
| Survey data | |
| Data output | The data stored and downloadable from the sensor consists of - Raw (multichannel) gamma-ray spectra; - Energy-stabilized gamma-ray spectra (in 10keV/channel format covering 0-3meV); - Radionuclide concentrations and their uncertainties (K, U, Th and Cs). Other nuclides can be added on request; - Complete GPS records (consisting of lat, long, elevation, DOP, No of satellites, quality of positions - Pressure, temperature and humidity. |
| Data format | Data is stored and streamed in (encrypted) JSON format. JSON definitions are defined on the mDOS JSON descriptionpage. Data contains: 1. Energy-stabilised multichannel spectra; 2. Nuclide concentrations; 3. GPS locations 4. Pressure/temp/humidity |
| Aux Data Format /streaming | Several add-on data formats compatible with 3rd party logging equipment are available on request: - MAVLINK - RMS - Geoduster - PicoDAS |
| Data protection | Survey data can be encrypted or left unencrypted. Encryption and decryption are done via proprietary algorithms embedded in Medusa processing software (GAMMAN). Unencrypted data storage and/or streaming are available on request. |
| Survey modes | 1. Autonomous streaming (preset timer) 2. Triggered streaming (external trigger) 3. Local (system stores data locally only). |
| Operating System | |
| Operating system | mDOS (Medusa Detector Operating System). mDOS provides a standardised web interface to our detector system, which allows set-up and check of system parameters and system functioning. |
| Physical | ||||
|---|---|---|---|---|
| Power | 9-36V, 6W max, 3W nominal | |||
| Sizes & weights | Type | Housing | Size | Weight |
| MS-350 Handheld/drone | Carbon fibre tube | 100 ⌀ x 210 mm | 2.7 kg | |
| MS-700 Handheld/drone | Carbon fibre tube | 100 ⌀ x 290 mm | 4.7 kg | |
| MS-1000 Drone | Carbon fibre tube | 100 ⌀ x 370 mm | 6.7 kg | |
| MS-1200 Underwater | Alu/stainless steel tube | 108 ⌀ x 1000 mm | 15 kg | |
| MS-2000 Car-borne | Carbon fibre tube | 120 ⌀ x 580 mm | 11.7 kg | |
| MS-4000 Airborne | Square aluminium box | 120 x 198 x 8500 mm | 23 kg | |
| NOTE: MS-50 and MS-100 sizes and weights are not listed as they vary depending on the application. Detailed dimension and drawings of the systems can be found on the system dimension page. | ||||
| Operating temperature | -10 oC to 50 oC |
1.2.1. Notes
(*) FPGA = Field Programmable Gate Array - a high-speed electronic component that can be tailored to a certain data processing job;
(**) Zero dead-time = no dead-time like in classic systems based on peak height determination and pulse conversion times. An "apparent dead time" is included by adjusting the live time clock for loss of system-measured pile-up rejections to ensure the absolute count rate is correct.
(***) The sampling speed of the FPGA and the scintillator output's peak width determines the system's theoretical count rate. The max data rate for real-world NaI or CsI crystals will be around the number mentioned.