The heart of the journey
20 Aug 2009




Once samples, accelerator, instrument and sample environment are ready, the experiment can begin.

Supriyo Ganguly and Dimitrios Bakauos (University of Manchester) using Engin-X for examination of residual stress around welds in an aircraft wing spar specimen

On arriving, visitors must first register at the User Office - or the Main Control Room if out of hours. “The students who come are often nervous and shy especially if they are from overseas. We always smile and try to make them feel at home,” says Emma Roberts of the User Office.

Safety first

Understanding safety is an important aspect of working at ISIS, so new users have to go through safety-training. This involves viewing an on-line tutorial explaining, for example, the interlock system of shutters that protects users when accessing experimental areas. They then take a test - a series of short multiple-choice questions - on the use of chemicals, alarm systems, radiation hazards. “They get three chances to pass. We’ve never had anyone fail - although some have taken the three attempts!,” says Emma. Each user is given a radiation monitoring badge and a swipe card for secure access to the experiment hall.

The instrument scientist will then meet the visiting team, and take them to the experimental cabin. “If researchers are inexperienced, I usually sit down with them and discuss their samples. We try to give them the largest-diameter neutron beam we can for a sample to get the best statistics,” says Steve King. “Students usually come with their supervisors, but if not, we spend time running through the experimental set-up and explaining the control software.”

Instrument scientists provide all the scientific support that the visiting team needs in order to get the best from their experiment.

The experiment

The SANS experiment being carried by Peter Griffiths and Paola Occhipinti required two days, with each of the 40 samples taking about an hour to run on the LOQ instrument. An automated sample-changer allowed the researchers to plan the programme and leave it running for 12 hours. While the first set of samples was running, the next day’s samples could be prepared.

“I used the ISIS laboratory facilities quite a lot,” explains Paola. “It’s nice because you meet lots of people and can exchange ideas.”   

Experiments are carried out over the full 24 hours to make full use of the Facility. The ISIS support staff for the instrument, sample environment, accelerator and beam-line are always available at the end of a telephone.

“It’s quite intensive and tiring; you don’t even know whether it’s night or day,” Paola comments. “It’s a very high pressure environment for a student,” agrees Peter. “It’s a chapter of the PhD thesis so the experiment has got to work.”

Collecting the data

Once the experiment starts, data are collected using specialist hardware and become available for the researchers to examine. The arrival of each neutron at one of the thousands of detecting elements comprising the detector area must be ‘time-stamped’. These events are counted and this raw data is grouped into histograms and transferred to the instrument’s dedicated computer.

Freddie Akeroyd is one of the computing group who maintain these computers and provide software to control the data capture. “Settings for a variety of parameters, such as sample temperature, can be specified in command files for overnight running,” says Freddie. “The software also allows you to look at the data live as it is collected, so you can quickly check if the instrument is working properly.” 

In a typical SANS experiment, the raw data can be reduced to meaningful results immediately using a ‘tried and tested’ computer program that has been developed over the years. Instrument scientists such as Richard Heenan and Steve King are on hand to give advice. “If necessary I’ll log on to the data at home in the evening and give the users a phone call. Sometimes I’ll come back in and we will go through the data-reduction process,” says Richard.

Reviewing Maps data of high-energy spin excitations in iron

Toby Perring (ISIS) and Andrew Walters (UCL/ISIS) reviewing their Maps data of high-energy spin excitations in iron
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The experiment gives a two-dimensional pattern of the scattered neutron intensity as a function of coordinates which can then be manipulated in a number of ways. In the case of Peter and Paola’s experiments, a plot is derived which reveals the required information about polymer conformation.

“We obtain just enough information to make any necessary adjustments to the next set of samples,” says Peter. When the final datasets are ready, they can be put onto a CD or memory stick and taken back to the user’s laboratory for further analysis. A copy of the data is also archived at ISIS.

Experiment control

Users need to be able to control their experiments easily, and this can be done remotely using a computer interface. This is where Kathryn Baker of the ISIS Computing Group steps in. She and her colleagues develop and monitor the software for manipulating all aspects of the experiment, including temperature, and movement of the samples and equipment. Users are presented with a visual display generated with a graphical programming language called LabVIEW. “It’s all ‘drag and drop’ with a mouse”, explains Kathryn.

Sample safety

An important practicality of neutron scattering is to check the sample for radioactivity once the experiment has finished. Neutrons can activate samples through nuclear reactions. If the dose rate is above a predetermined level, a member of the ISIS health physics section collects the sample for storage in shielded locations. “Activated samples usually decay rapidly,” says Paul Wright who is the ISIS health physicist and radiation protection officer. After a few days, the sample can normally be released for return to the user’s laboratory.

Next: At journey's end