6.3.3 Photomultiplier tubes and detection circuitry

The visible photons are collected by an array of photomultiplier tubes behind the crystal. These convert each visible photon to an electron and then multiply the number of electrons sufficiently to give a voltage pulse. Because the number of visible photons is proportional to the energy of the incoming gamma ray, the height of the pulse depends on this energy. This gives a method of counting the numbers of gamma photons at different energies that reach the crystal.

A resistive network c
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

2.2.4 Film cassette and grid

As the X-rays pass through the patient some of them will be scattered and will therefore not follow the expected line through the patient. If these reach the detector they will blur the image. Some of the scattered radiation can be removed by a grid, usually oscillating, placed between the patient and the detector.

Analogue imaging systems use either film alone (rarely) or a combination of a film and fluorescent material (phosphor). The phosphor fluoresces and produces visible light whi
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

2.6 End-of-unit questions

Question 1

What is the approximate wavelength range (in metres) of microwaves?

Learning outcomes

By the end of this unit you should be able to:

  • identify the common features shown by tree-dwelling mammals from different groups;

  • show an awareness of the difficulties of classifying primates, especially in relation to the position of the prosimians;

  • give an account of opportunities and challenges encountered by tree-dwelling mammals and of evolved adaptations linked with arboreal life;

  • provide examples of the closeness (and sometimes
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

2 Variable structure of ape societies

You will be aware from watching the TV programme that information on behaviour and social structure of apes is gathered by many hours of patient observation.

Activity 2

4.2 Other members of the bear family

Other omnivorous species of bear include the Asian black bear, the North American black bear and the Andean spectacled bear. Although polar bears spend their winters hunting seals out on the Arctic sea-ice, they have to come ashore when the ice melts in spring and find other sources of food.

Activity 2
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

6.2.4 The speed and direction of the Earth's motion

The first significant claim to have detected the motion of the Earth relative to the ‘frame of isotropic 3 K radiation’ came in 1977 from a group at Berkeley, California. They concluded that the Earth is moving at a speed of (390 ± 60) km s−1, in a direction towards the constellation Leo, relative to a frame in which the 3 K radiation is isotropic. Their conclusion resulted from observations of a variation of intensity with angle of the form predicted by Equation 14, which w
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

5.3 The redshift of the 3 K radiation

The temperature, T, of the radiation is proportional to the most probable photon energy, E, which as we have said is proportional to f, and hence inversely proportional to the wavelength λ. Thus,

According to Equation 1, we have for the redshift, z


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

4.2 Evidence for a big bang

Having interpreted the redshift as indicating a recessional speed proportional to distance, one may extrapolate into the future to predict how the positions of the galaxies will evolve with time. One can also run the sequence backwards, so to speak, to discuss what their positions were in the past. Clearly, at former times the galaxies were closer to each other.

But not only that. Because of the proportional relationship between speed and distance (Equation 6), at a certain time in the
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.2 Some general properties of galaxies

Firstly, we note that galaxies tend to occur in clusters rather than singly. The mutual gravitational attraction of galaxies naturally tends to hold them on paths that remain close to each other. Typically a cluster contains tens or hundreds of galaxies. There are, however, large clusters with thousands of galaxies, and there are some solitary galaxies. Our own Galaxy is a member of a smallish cluster of about 36 galaxies called the Local Group (see Author(s): The Open University

1 Introducing cosmology

General relativity has a very different conceptual basis from that of Newtonian mechanics. Its success in accounting for the precession of Mercury's orbit, and the bending of light by massive objects like the Sun, gives us confidence that our picture of space and time should be Einstein's rather than Newton's. In this and the following units, we turn our attention to the study of the large-scale structure of spacetime. We see how spacetime as a whole is curved by the gross distribution of mas
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

5.1.5 Getting agreement with the Ampère–Maxwell law

Finally, our electric and magnetic fields must satisfy the Ampère–Maxwell law in empty space. Using Equations 7.21 and 7.23, we obtain

which requires that

Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

4.5 Endocrine disruptors

Then he was a she…

(Lou Reed, American rock singer)

In 1996, a book called Our Stolen Future was published, bringing to public attention a debate that had been simmering among biologists for some time. Written by Theo Colborn and two colleagues at the World Wildlife Fund (WWF), this book presented the hypothesis that certain industrial chemicals, commonly found as environmental pol
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.2 The impact of climate change on global freshwater resources

The availability of freshwater will be significantly altered in a future world affected by climate change (Houghton, 2004). In some regions, water availability will decrease; in others it will increase. Precise predictions about the extent and exact location of such changes cannot be made because they are based on climate models, the accuracy of which is uncertain. However, there is wide agreement that probable changes will include:

  • More rain in north
    Author(s): The Open University

    License information
    Related content

    Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.6.1 Radioactivity and bugs!

Many natural processes involve repeated doublings or halving at regular intervals. You may have come across this already in your work, in the context of bacterial growth or radioactivity. In this section, we are going to look in more detail at bacterial growth and radioactivity and we will be using graphs to examine how the numbers of bacteria or numbers of radioactive atoms change over time.


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

3.2.2 Choice of scale

It's important to choose a scale that covers the range of values you have recorded for that particular axis. If the scale is too big, then all of your measurements will be bunched up at one end of the graph, making it difficult to read. It is also very important to keep the scale consistent all along the axis, i.e. don't suddenly change the spacing between the units of measurement on an axis.


Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share

7 Unit questions

Now you have completed this unit, try the following questions to test your understanding of this material.

Question 19

11.1 The relationship between frequency and pitch

Although the perception of sound involves the interaction of frequency and intensity, many aspects of frequency reception can be analysed separately.

For normal or typical hearing, the limits of hearing for frequency fall between 20 and 20 000 Hz. Below 20 Hz only a feeling of vibration is perceived; above 20 000 Hz, only a ‘tickling’ is experienced.

As well as loudness, the other most obvious characteristic of a sound is its pitch. Pitch is a subjective dimension of he
Author(s): The Open University

License information
Related content

Except for third party materials and/or otherwise stated (see terms and conditions) the content in OpenLearn is released for use under the terms of the Creative Commons Attribution-NonCommercial-Share