We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.
We use cookies to distinguish you from other users and to provide you with a better experience on our websites. Close this message to accept cookies or find out how to manage your cookie settings.
Published online by Cambridge University Press: 19 August 2009
The three axes involved in a triple-axis instrument are the monochromator axis, the sample axis and the analyzer axis as shown schematically in Fig. 3.1. However, there are many other elements of a three-axis instrument which are necessary in order to have an efficient spectrometer. These include monochromator and analyzer crystals, energy filters, collimators, and detectors. Since each axis and crystal has to be individually moved for every setting of the instrument, computer control is essential. In addition, extensive shielding is required in order to protect the experimenter and to reduce the overall background or noise in the experiment. In this chapter we shall discuss each of these elements in some detail.
Shielding
Most modern reactors have beam tubes which do not look directly at the core, but instead have their axes tangential to it. This decreases the quantity of unwanted high-energy (≳ 200 meV) neutrons (also called fast neutrons) in the beam tube, since only those neutrons that have a component of velocity nearly parallel to the beam-tube axis can enter the monochromator area. Neutrons which satisfy this condition have necessarily undergone collisions with the moderating material and have lost considerable energy compared to fission energies. The moderation is never perfect, and there are always some fast neutrons that enter the monochromator area, along with γ-rays and other unwanted radiation. It is therefore necessary to shield adequately against this unwanted radiation. The shielding should contain a combination of materials that can scatter the fast neutrons, slow them down, and then absorb them. In addition, a significant amount of lead is needed to absorb the dangerous γ-rays.
To save this book to your Kindle, first ensure no-reply@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Find out more about the Kindle Personal Document Service.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Dropbox.
To save content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about saving content to Google Drive.
