How to Use a Cryostat

A cryostat is a vacuum-insulated vessel with a sample environment that can be cooled to cryogenic (very low) temperatures, typically measured in Kelvin. Cryostats can use liquid cryogens, such as Nitrogen or Helium, or a mechanical cooler to achieve low temperatures. Samples are mounted either on a Copper heat exchanger or placed in a sample tube filled with cooled gas (typically Helium). The outer vacuum casing protects the sample. There is often a second, inner radiation shield to further insulate the sample from radiated heat, helping to maintain the lowest temperatures.

Cooling is achieved by either flowing liquid cryogens through the copper heat exchanger or by mechanical contact with a mechanical cooler such as a Gifford McMahon (GM) cooler.

The temperature is controlled by balancing the cooling effect of the flowing cryogen with a voltage across a heater and by monitoring the temperature of the sample with a sensor.

You can vary the voltage across the heater and then increase or decrease the flow of cryogen to achieve a balance and maintain temperature stability. This is usually automated with the use of an electronic temperature controller. Temperature controllers use a simple PID loop to achieve temperature stability and manage temperature ramping, minimising temperature overshoot and maximising the efficient use of the cryogens.

The two major cryogens used are liquid Nitrogen, which is cheap, easy to source and gives you a base temperature of 77K, or liquid Helium, which has a boiling point of 4.2 degrees Kelvin or 4.2 degrees above absolute zero.

By using vacuum pumps to reduce the pressure over the liquid, you can decrease the temperature further to about 1.5 degrees Kelvin for liquid Helium. Cryostat designs also exist that use 3He or a mixture of 3He and 4He to achieve temperatures as low as 10 milliKelvin.

Liquid Nitrogen Bath Cryostats

These types of systems are straightforward and simple to use. First, we need to evacuate the isolation space – the space isolating the sample from the outside world. The air is pumped out using a turbo pump to reach a base pressure of 10^-4 to 10^-6 mbar. Once the vacuum space has evacuated, the cryostat can be cooled, usually by pouring liquid Nitrogen into the bath using a funnel. Once cold you can connect the temperature controller and set the temperature for your sample space. The Nitrogen flow is gravity-fed to the heat exchanger using a thumbwheel connected to a needle valve. Opening the valve will create more flow and cool faster. Closing the valve down will reduce the flow to maintain set temperatures. The set temperature and stability is maintained by the temperature controller, which varies the voltage across the heater according to the sensor reading at the sample position.

Liquid Helium Systems are a Little More Complicated

With liquid Helium systems, again the vacuum space must first be evacuated using a turbo pump or similar. Helium is transferred to the cryostat from a storage vessel (dewar) using a vacuum insulated transfer line. A small flow pump is used to draw the helium through the system. Once the “spent” helium gas has passed through the cryostat it is usually vented to a helium recovery system where it can be cleaned, compressed and re-liquified for re-use.

Helium flow can be controlled either manually with a needle valve or, in some cases, the temperature controller can vary the flow using a stepper motor connected to the needle valve. Again, voltage is varied by the controller to maintain stable set temperatures. The use of automated gas flow can vastly improve the cost and efficiency of the cryostat by minimising the volume of helium required to maintain the target temperature.

Health and Safety Considerations

There are important health and safety considerations when using liquid cryogens. Firstly, cryogens cause burns. You will never see liquid Helium in the atmosphere because it can't exist as a liquid above 4 Kelvin. You can see it inside a cryostat with glass windows, but outside of the cryostat it will never be in liquid form. It's very, very cold. Anything that has been cooled by liquid Helium will be cold enough to cause burns as skin sticks to the cold surface. Serious burns can occur faster than your ability to react to contact.

Oxygen and air near cold surfaces can become Oxygen-enriched as Oxygen liquefies in contact with anything at those temperatures. This can pose a fire hazard, especially in the presence of oil or grease on nearby surfaces.

Asphyxiation hazards: If you are using liquid cryogens in a laboratory or room, storage dewars or cryostats will slowly boil-off Nitrogen or Helium gas. One litre of liquid Nitrogen will evolve into 690 litres of gas at room temperature. Over time this has the potential to displace the air in the laboratory resulting in a lack of Oxygen and danger of death. It is imperative to ensure good ventilation, especially in enclosed spaces such as optics labs. The use of oxygen depletion monitors is also highly recommended. These can be fitted into the laboratory and/or carried by all staff who enter.

Can You Damage the Sample?

It depends what the sample is. Thermal shock is a risk for delicate samples. It is important to consider how quickly the sample will be cooled down and warmed up again. There is the presence of Helium, which, though inert, may affect your sample if it is prepared in another gas, or a vacuum. Then there is moisture. If you take a cold sample out before it has thoroughly warmed up, water will condense on the sample, which could damage it.

What Type of Experiments are Performed with Cryostats?

We can study the electrical and physical properties of materials that change with temperature. We might want to cool the environment down so that our sample signal can be dominant – increasing the signal to noise ratio. We may also wish to observe electrical transition states at different temperatures.

We can use cryostats for infrared spectroscopy for polymer research, inorganic chemistry, medicinal chemistry and solid state and semiconductor physics, for example. Raman spectroscopy is used to analyse organisms and tissue and UV-Vis spectroscopy in analytical chemistry.

It is easier to detect photoluminescence and fluorescence at low temperatures. Photoreflectance is used to look at the electronic structure of semiconductors. Andor's cryostats are used in a huge range of applications around the world.

What is Cryostat Sectioning?

This is when people take biological samples and slice them into very thin sections, analyze them and do assays. You would use large cryogenic fridges to store biological samples or liquid nitrogen vessels. This has nothing to do with our optical cryostats!

How Do You Cool a Liquid Sample?

For liquid samples we supply systems with cuvette holders which can hold stoppered cuvettes. They can be put into cryostats that use an exchange gas sample environment. Exchange gas cryostats have a sample tube that is filled with a small volume of Helium gas. This gas is cooled by the cryostat. By inserting samples into the gas filled sample tube you can cool liquids, powders or solids. Cold gas cryostats are also good for cooling samples with poor thermal properties, bathing them completely in a cold gas.

How Do You Cool a Powdered Sample?

On standard sample holders, even in vacuum sample spaces, it is possible to compress powders into pellets under high pressure and then mount those pellets into a sample holder. Alternatively, bulk powder samples can be held in cuvettes, as with liquid samples.

Things to be aware of are:

• Good health and safety
• Good PPE
• Good preparation – have everything ready before you bring the cryogen to the cryostat particularly with Helium systems.
• Be aware and careful. There are other risks such as having glass windows under vacuum. It is possible to be clumsy and break a window so you lose your vacuum. The liquid cryogens in the system will boil very quickly and the problems will snowball. You need to be aware and careful.

Where Do Andor’s Cryostats Excel?

Andor's product range excels in its very efficient use of cryogens. Our helium cryostats achieve low temperatures with a minimal use of Helium. Our low loss transfer tube technology uses the spent gas to precool the radiation shields in our transfer lines. This improves transfer efficiency and reduces helium losses. This means that the cost of ownership of our cryostats is very low. The initial outlay can be a little more, but over one or two years the cost of operation can typically be half that of a competitor’s system. In the medium and long term the real cost of ownership of our systems can be considerably better.

Our Optistat Specification Sheets

Full details of our cryostat range can be found on our specification sheets, available on our website. We have a range of cryostats using Nitrogen, Helium or GM coolers.

An example is OptistatDry^TM which is designed for ease of use with free-space optics. You can very precisely align your cryostat and sample position, changing samples in and out numerous times without moving the cryostat or disrupting the alignment of your experiment. Changing samples with minimal impact on optical alignment can be time saving and maximise efficiency.