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Pomelo: Hand-held gamma-ray spectrometer

Battery-powered instrument for gamma spectroscopy with ample interfacing options

mihaicuciucmihai.cuciuc
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Pomelo is a stand-alone instrument for gamma-ray spectroscopy. It uses a CsI(Tl) scintillator to measure gamma ray energies and can be operated using its buttons & graphical LCD interface. The hardware also supports USB, WiFi, Bluetooth, ZigBee, and an SD card. The whole thing is powered by a rechargable 18650 battery.

Pomelo is a complete instrument for hobby-level gamma spectroscopy. It is built around the Pomelo Core and Physics which handle all the radiation detection, but adds a user interface and ample communication in the form of an Arduino-compatible ESP32-C6 add-on board, the Pomelo Zest.

Pomelo Zest board displaying a Th-232 spectrum in logarithmic scale

The Pomelo Zest features:

A brief user interface demo of the instrument is shown in the following video.

The Pomelo Zest can be used as a generic ESP32-C6 development board, but its mechanical design is built to match the Pomelo Core and Physics so it might not be optimal because of the battery and power button placement.

Block diagram

Detector block diagram showing available connectivity for the system components. The Pomelo Core can still be accessed from a computer through USB for control and firmware update even while it also connected to the Zest

The software on the Pomelo Zest is written in the Arduino environment so it is very easy to follow and modify, but not optimized for low power consumption. However, the Pomelo Core stays powered when the Zest is switched off, so data can still be accumulated using little power.

Schematic

Pomelo Zest schematic

  • WiFi connectivity

    mihai.cuciuc • 08/19/2024 at 05:09 • 0 comments

    Pomelo Zest offers WiFi connectivity thanks to the ESP32-C6 at its core. It can either connect to an existing network or run in access point mode to allow a phone or a laptop to connect to it for accessing the web user interface.

    An overview of this functionality is in the following clip, with some more detail below.

    To make connection as easy as possible it can display the WiFi credentials (when in AP mode) and web UI link as QR codes ready to be scanned by a phone

    Web interface as presented on a laptop and a phone

    The web user interface is pretty basic, having run information and controls as well as a spectrum view implemented using the very tiny and awesome uPlot library. This allows having all assets locally, as you might not be online when connected to Pomelo.

    We gamma spectroscopy nerds really care about poking at the gamma spectra looking for peaks and making sure the energy resolution of our instrument is as advertised. A lot of excellent software has been written for this and one of my favourites is InterSpec. Pomelo offers a minimal N42 file as a download with the current spectrum and its calibration coefficients, such that it can immediately be loaded into InterSpec.

    InterSpec showing gamma spectrum loaded up from Pomelo

  • Radiation mapping

    mihai.cuciuc • 07/29/2024 at 17:59 • 0 comments

    Check out the interactive version of the map here: http://pomelomap.s3-website.eu-central-1.amazonaws.com

    I strapped a GPS module to one of the Pomelo Zest prototypes and drove it around for the past week. It logs positioning and spectrum data to text files on an SD card and I then read all those text files in a python script to generate a map using Folium. Individual measurement points are grouped together in squares on the map and all the spectra from these measurements are added together to calculate an average dose rate at that point. You can click a square to see the associated spectrum.

    GPS module jerry-rigged onto detector

  • Updated scintillator and power consumption

    mihai.cuciuc • 06/19/2024 at 17:30 • 0 comments

    Pomelo detector on the shoulders of giants

    I got the scintillators I intended to use with this detector and have some performance plots with them in this project log. With them the hand-held detector is also starting to look better. I added top & bottom laser cut acrylic plates as basic protection of both electronics as well as of fingers from the ~80V generated by the bias circuit.

    The detector consists of a top user interface & connectivity board based on the ESP32-C6 (Pomelo Zest), and a bottom gamma spectroscopy board (Pomelo Core). The power consumption of these two boards can be measured individually:

    Pomelo Core:

    • active: ~3 mA at background radiation levels

    Pomelo Zest:

    • active, connected to WiFi: 50 mA
    • active, LCD backlight on: 70 mA
    • active: 40 mA
    • light sleep: 0.7 mA
    • deep sleep: 0.35 mA
    • deep sleep with SD card inserted: 0.65 mA

    On future board revisions I intend to gate the power to the LCD and SD card.

    Here are some pictures of the detector with its laser cut acrylic protection plates.


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Discussions

kirimayne12 wrote 11/05/2024 at 20:00 • point

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Gianmarko72 wrote 06/13/2024 at 08:02 • point

this is a very interesting project.
is it going to be available in some form like kit?

  Are you sure? yes | no

mihai.cuciuc wrote 06/19/2024 at 16:17 • point

Hi! Yes, that is the plan :) But it might take a while until I'm actually ready to sell these. Hardware is pretty advanced, but getting all the paperwork in order is something I'm not very experienced with.

  Are you sure? yes | no

Gianmarko72 wrote 06/25/2024 at 06:29 • point

Hi Mihai
we will patiently wait :-)

  Are you sure? yes | no

kogosi wrote 06/07/2024 at 07:02 • point

Hi, i´m Ralf. What Type ist the scintillator? I dont find anything in the Net. Thanks. Ralf from Germay ;-)

  Are you sure? yes | no

mihai.cuciuc wrote 06/19/2024 at 16:15 • point

Hi, Ralf! I used a CsI(Tl) scintillator from OST-Photonics: https://www.ost-photonics.com/product/10-mm-x-10-mm-x-30-mm-csitl-scintillation-crystal/

It's 3 cm^3, but I've since switched to 2 cm^2 ones :)

  Are you sure? yes | no

Gianmarko72 wrote 06/25/2024 at 06:30 • point

Hi Mihai
why move to smaller scintillators? i thought the bigger the better (more chances to catch an event)

  Are you sure? yes | no

mihai.cuciuc wrote 06/25/2024 at 06:45 • point

Hi, Gianmarko! The main reason I moved to smaller scintillators was that I read in someone's thesis that the reflective wrapping I use for the crystal works best when the aspect ratio of the crystal was at most 2:1. The previous crystals were 30 mm long with a base of 10 mm x 10 mm. Another reason is cost, as the price of the crystal goes quite linearly with size. Now the crystal and SiPM are in the same ballpark :)

  Are you sure? yes | no

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