Metrology : an overview
Just what is meant by the word “metrology” in this context?
A fair question indeed! Breaking the word into its components, we find that it means “the study of measurement”. Broadly speaking, in this context of scientific experimentation, how we measure things- the techniques, the principles and practice, the limitations- of measuring what we need to measure in order to undertake and accomplish the research we are doing.
With regards to the Centre’s interests in detecting dark matter particles, we might consider this to encompass two main regimes:
unwanted, but unavoidable, items which introduce a background signal which we must understand and account for;
some desired experimental signal which we really do want to be able to observe and to measure accurately and precisely, usually (if not always) in the presence of some sort of background as mentioned above.
Further, given the nature of the experimental work with which the Centre is concerned, we are concerned here with precision metrology - where the quantities to be measured must be measured very precisely (that is, our measurement of some observable must be sufficient to reveal its true value as nearly as possible), and where we may be dealing with very minute amounts of something, such as parts per billion of some contaminant.
Let us approach the matter of backgrounds first.
There are times when scientists are fortunate enough to be interested in measuring some quantity where the signal- that quantity, or variation in some quantity, that we wish to determine- is very clear and strong, and there is no doubt about what we’re measuring, and no difficulty in getting that measurement.
In such cases, the background signal or signals (the stuff we don’t want, and which may obscure the desired signal, but which we cannot avoid) is small by comparison, and, perhaps, no great care is required for it to be dealt with.
Many other times, however, that is not the case… we may be looking for a signal which seems swamped by noise, where background signals are much stronger than that which we seek, and we must take great care to eliminate as much background as we can, and measure what remains as well as we can, such that we may understand it and remove its effects from our data as well as we possibly can.
It is within this second broad scenario- small signals, and great requirement for the understanding of the background/noise components of our data- that our experimental investigation of dark matter particles lies; and that is why an ability to undertake precise measurements is vital- and this includes precise measurements of the backgrounds and whatever produces them.
As an examples of backgrounds, let us consider the kind of direct detection experiment that constitutes SABRE.
Here, we are looking for flashes of light produced within a copper-encased, thallium-doped NaI crystal, as well as flashes of light within a liquid scintillator bath that surrounds the crystal-containing copper tubes. Ideally, we are interested in a light flash only within the crystal- this is our desired signal, and it would indicate the possibility of a dark-matter particle interacting with the crystal.
The surrounding liquid scintillator is used as a veto; if we see a flash in the scintillator at the same time as a flash in the crystal, the inference is that both flashes probably came from something other than a dark-matter-related event.
One major concern here is that the very material of which the detector is made- the crystals themselves, the copper, the steel, almost anything- can release particles that will interfere with the detection and veto process. Further, the surrounding environment can do the same- for example, cosmic rays from space, and natural radioactivity in surrounding materials in the lab. These constitute and produce the background against which we try to measure the desired signal.
Thus, an important part of the SABRE experiment is to use materials which have the lowest possible amounts of background-producing contaminants- and we need precision measurements of these contaminants in order to produce the lowest-possible background level, and also to know how much that background will influence our measurements.
As an indication, we are often talking about contaminant levels and measurements of the order of parts-per-billion.
As an example of signals, let us have a look at an experiment such as the ORGAN experiment at the University of Western Australia.
ORGAN is an axion haloscope, and even one of the largest such detectors in the world (ADMX) operates at expected signal strengths of the order of 10-22 Watts. (Imagine how dim an old-fashioned 1-Watt light globe would be… and then around one million-million-millionth of that… then divide that by ten-thousand. That's not a lot of power!)
Low-noise amplifiers are then used to boost that signal, but the point is that we are still concerned with dealing with extremely low-strength signals, and with coming up with ways to measure those very faint signals without adding so much noise that the signal is swamped.
And that takes some very fine and careful engineering, including ways of measuring such minute quantities.
This includes cooling the parts of the apparatus to reduce thermal effects- not just to a temperature that’s a tad chilly, but down to around 4 degrees Kelvin; that’s four degrees above the coldest temperature possible anywhere in the universe. Not only does that refrigerator have to get things down that low, but the achieved temperature must be measured precisely and kept stable.
So, we see that precise measurement- precision metrology- is a basic requirement of the search for dark matter particles. It absolutely underpins the experimental methods used.
We will look at various aspects of this in later posts.