The Forever Dosimeter

The Forever Dosimeter Realtime Radiation Detector

There are currently many dosimeter badge manufacturers. Some designs use crystals that are heated in a special oven. The badge is worn throughout a work day and is then collected and placed in a specialized reader to determine the radiation dosage accumulated throughout the day. The advantage of such a design is the lack of batteries that can run down or wear out. The disadvantage is the need for trained personnel to disseminate and collect the badges and run the equipment to determine any radiation detected. These badges do not allow for real time dosimeter readings.

Another type of badge uses an embedded MCU and radiation detector chip or circuit. These badges do run off of batteries and most allow real time dosimeter readings. The longest claimed battery life that has actually been validated is two years, with the average battery life of these designs at just over six months. This requires ongoing maintenance to check and replace the batteries when they decline with age or run down due to usage of the devices.

The Forever Dosimeter design does away with batteries entirely. This badge uses a solar cell optimized for indoor charging with a miniaturized super capacitor that uses the solar cell as a power source. Since human beings generally do not work in the dark, this design provides a battery free badge powered by ambient lighting whose lifetime is measured decades not months or years. A batteryless design also means that no personnel will be tasked with monitoring and replacing batteries since there are none to replace.

The Forever Dosimeter is a Bluetooth Low Energy (BLE) device that is read by stationary readers placed around a facility or by a user's smartphone. If badge coordinates are required they can either be determined by Bluetooth positioning from stationary Bluetooth readers of the dosimeter itself, the Bluetooth positioning location of the smartphone reading the badge, or via the smartphone's GPS.

Varadis Radiation Sensor

A RADFET is an exteremely low power radiation dosimeter chip that uses just 10 microamps of current to detect radiation exposure. The VT01 is the first generation RADFET chip from Varadis and comes with dual RADFET devices on a single 6-pin SMD chip. A RADFET is a field effect transistor that is sensitive to gamma radiation and requires no power for sensing and extremely low power for reading.

The chip is run in one of two modes: detection or readout. The detection mode requires no power at all. The chip simply absorbs gamma radiation which changes the electrical properties of the RADFET devices within.

For readout a voltage is placed across the field effect transistor with a constant 10 microamp current and is read by the to the nrf5340's on-chip ADC. The voltage drop is adjusted by temperature to give a radiation exposure result.

Epishine Indoor Solar Cell

Epishine produces a thin film solar cell optimized for indoor lighting as a power source. The epishine cell is flexible and can be bent or flexed without damaging the cells within.

Epishine produces an EVK complete with a Cap-XX GA230 Supercapacitor and an E-peas AEM10941 power management IC. This is the kit used to test and develop the Forever Dosimeter badge with the Nordic Semi nRF5340-DK development kit.

Cap-XX Super Capacitor

Cap-XX is an Australian company that produces a line of miniature super capacitors. The one chosen for this project is the GA230 product. The GA230 mounts along two slightly raised rails on the PCB. Cap-XX has recently inroduced an SMD version of this component that can mount directly onto PCB traces but this product was unfortunately not available for the first Forever Dosimeter prototype.

u-Blox nRF5340 Module

Developing a 2.4 GHz impedance balanced circuit with antennas is not a trivial task. Additionally FCC certification of a completely new design can be costly and time consuming. There are several companies who build pre-certified modules where only antenna changes, if any, need to re-certified. This is a cheaper and faster alternative than certifying a complete ground up design.

I chose the new u-Blox Nora-B106 module which comes complete with an embedded antenna and is pre-certified with the FCC and CE. This avoids having to interact with an approved certification laboratory and reduces the expense needed to get a new design off the ground considerably.

Unfortunately the module is on back-order with all of the suppliers I contacted so this design is currently waiting for parts in order to be fully prototyped and tested.

Design Testbed

Circuit Diagram

The dosimeter circuit consists of three main components;

1. Power section from solar & super-capacitor and converting it to 3.3v

2. The nRF5340 MCU & Bluetooth module

3. The RADFET radiation sensing circuit

The solar cell & super capacitor circuit is centered around the e-peas AEM10491 power management chip that receives power from the Epishine indoor solar cell and stores it in a Cap-XX GA230 super capacitor. An optional CR2302 battery backup is available but not required for normal operation in a normally lit indoor environment. The e-peas chip converts the energy stored in the super capacitor to a regulated 3.3 volts for the nRF5340 MCU.

The MODUL_IN label is a direct connection to the Epishine solar cell while the Cap-XX GA230 super capacitor is on the bottom right of the schematic. The e-peas power management chip produces a regulated 3.3v at HVOUT.

The Bluetooth circuit is a self-contained pre-certified module from u-blox complete with a chip antenna. The u-blox module feeds directly off of the regulated 3.3v output from the e-peas AEM10941 chip.

The RADFET chip from Varadis is the VT01 which is an 6-pin chip with two RADFET radiation sensitive field effect transitors on board. In sensing mode both RADFETS are connected to ground. In readout mode they are connect to a 20v voltage source at 10 microamps where the temperature adjusted voltage drop determines the level of radiation exposure. The regulated 3.3v from the e-peas chip is stepped up to 20v by a TL499A DC-DC boost chip. The 20v output is fed through an LM334 current limiter to product a 10 microamp output. The RADFETs are switched from grounded inputs to readout mode by a dual SPDT chip. The output voltage is stepped down to below 3.3v and fed to the ADC's on the nRF5340.

The Varadis VT01 is a 6-pin SMD chip with two RADFETs on die.

The VT01 pins are tied to ground when sensing and they are switched to 20v at 10 microamps for readout. Radiation exposure is sensed as a voltage drop across the gate to drain of the field effect transistor.

Conclusion

The Forever Dosimeter uses novel technology with an extremely low power radiation sensor on-a-chip, a solar cell designed for indoor use, a super capacitor for energy storage, and an ARM Cortex-m33 MCU to create a self-powered radiation dosimeter with at least a 10-year lifespan. This design is the basis for a wearable badge or a mounted sensor that requires no maintenance or battery changes during its lifetime. Readout of the sensor badge is via Bluetooth to a smartphone or bluetooth receiver. The position of the badge or mounted sensor can be determined by the receiver using Bluetooth positioning.