• Question: What is it like to work with lasers? And how does cryogen work?

    Asked by Chris to Natalie on 19 Nov 2015.
    • Photo: Natalie Garrett

      Natalie Garrett answered on 19 Nov 2015:

      When you work with powerful lasers like I do, you have to undergo training before you’re even allowed to turn one of them on. This is for your own safety, so you know how to use them properly. The laser pointers you might have come across at school, by contrast, are not so dangerous. They are not allowed to be powerful enough to cause your eye damage, and are limited to 1 milli-Watt of power to be legally compliant in the UK.

      Powerful lasers can be dangerous for your eyesight, because the laser beams are so narrow and have so much energy in them. So, when the lens in your eye focuses the beam to a teeny tiny spot on your retina, the huge amount of energy dumped into such a small region can be enough to burst your eye, or cause irreparable damage. However, if you’re working with lasers that can be dangerous, you get given protective goggles that stop the beams from reaching your eye, so you’re perfectly safe as long as you are sensible and abide by the rules.

      As for cryogens, they are very, very cold materials. In our department, the cryogens we use are dry ice (the solid form of carbon dioxide, which is -80 degrees Celsius) liquid nitrogen (which is -196 degrees Celsius) and liquid helium (which boils at -270 degrees Celsius). It takes a lot of energy to cool carbon dioxide, nitrogen and helium so that they reach these types of temperatures. This is achieved using something called a cryocooler, which is a bit like a freezer but a lot more efficient and at much lower temperatures. Once your cryogen is at the temperature you want it, you then have to store it in something that will keep it cold, so it needs to be very well thermally insulated.

      When you want to use it (for instance, if you wanted to freeze a fresh piece of brain, you’d want to do it quickly to stop ice crystals from destroying the cells inside it, and this requires something very cold like liquid nitrogen) you have to be careful that you don’t release too much of the gas form of the cryogen. Nitrogen gas could displace all the air in your lungs and make you die of oxygen starvation if you weren’t careful! The density of liquid nitrogen is around 900 kg/cubic meters, and the density of nitrogen gas is 1.2 kg/cubic meters. Therefore when liquid nitrogen heats up and becomes a gas, the increase in volume is around 750 times! This means you have to be sure not to use too much of the liquid for the space you’re working in, otherwise if you dropped it you could push all the air out of the room and then this would kill you.