A while ago I was traveling through Riga airport where in the middle of the terminal I saw a machine I had never seen at any other airport. It looks like it might be related to security, however there was nothing around it that explained what it is. What is the purpose of this machine?
This is a detector of radioactive materials (to make sure passengers are not carrying anything radioactive), possibly model "TSA PM700" from Rapiscan Systems.
From the site:
A high sensitivity walk-through radiation portal monitor to automatically scan pedestrian traffic for radioactive materials.
- High Sensitivity Portal Monitor
- Continuously Scan Moving Pedestrians
- Gamma and Neutron Radiation Detection Options
- Fast, Seamless Integration
To backup the already correct answer, what you're looking at is a Radiation Portal Monitor. They vary in size depending on what they're scanning. They are designed to scan for radioactive materials. I've seen car-scale versions of these at the border-control in Calais, most likely acting on behalf of Project Cyclamen with more info found here.
Funnily enough these can apparently be set off even by recent medical procedures as described here:
My parents were on a coach that set one off. One of the passengers had been treated with radioactive iodine for thyroid cancer."
Although a better question to ask is why does Riga believe people are transporting radioactive material through an airport.