How does light deliver it’s energy?
In discrete packets- Photons, the original quanta
What is the photoelectric effect?
It is the emission of electrons from a material when it is bombarded with light or, more generally EM radiation.
What are the 3 observations associated with this effect?
- Light must be above a certain frequency for emission.
- The kinetic energy of the electrons is proportional to the frequency, if above threshold.
- Above threshold, current is proportional to intensity.
- Emission is instantaneous.
What is the work function of a material?
The minimum value of energy needed to emit an electron.
What can you measure using the Photoelectric effect?
Planck’s constant, h.
What does a photomultiplier do?
It amplifies the initial photoelectron pulse by a succession of diodes.
What explains the observations associated with the photoelectric effect?
Light consisting of discrete packets of energy called photons and materials having a work function.
What is a photocell? And what does it do?
They deliver a photocurrent when put into a circuit. A photocell is housed in a glass tube.
What is a photodiode?
A solid-state device generating electrons in response to light.
What is a photomultiplier used for?
To detect weak light sources; X-rays, Gamma rays.
Describe the two stages of detection in a photomultiplier?
- Radiation produces light vis Phosphors.
- Light is then detected by a photomultiplier, then amplified.
What does a photomultiplier consist of?
A photo multiplier is a larger glass tube than the photocell, it has a photosensitive surface that emits electrons, the photocathode, followed by a series of metal plates called dynodes.
What is image orthicon based on?
Based on photoelectric emission.
What does an image intensifier aim to do?
To give an electronic version of a picture, (this technology is behind the television).
What is image orthicon?
Photoelectrons create a positive replica of image on the internal plate(photomultiplier), the image is then scanned by an electron beam and scattered electrons are measured.
What is a CCD?
A charge coupled device, is a light sensitive integrated circuit which captures images by converting photons to electrons.
Describe the detection process of a CCD detector?
- Expose the light, generating the photoelectrons.
- Store the photoelectrons in electrical ‘wells’ directly beneath their place of generation.
- Transfer the stored charges to the edge of the device.
- Read-out the charge and generate a suitable digital number to represent the light irradiance.
What are the advantages of a CCD detector?
- Intrinsically digital.
- Highly linear
- Large dynamic range (the maximum to minimum levels that can be detected)
- Capable of long integration times (Enables imaging of sources too weak to be seen by eye)
- Low ‘noise’ (dark signal)
- It has a high quantum efficiency (success at creating a signal for incident quanta), the conversion of photon energy into electrons, is very high.
Describe how light is detected in the eye?
It is the retina in the eye that detects light, the retina has rods and cones. The molecule rhodopsin in which winds up and down a rod, contains the chromophore retinal which absorbs light. Upon absorbing the light retinal changes from the cis to the trans configuration, releasing an electron. This is a cascading process which leads to the generation of an optic nerve signal.
What is Retinal?
A chromophore contained in Rhodopsin that absorbs light.
Assuming you wished to use a photomultiplier to detect X-ray photons, describe the components of the photomultiplier and how they would function to allow you do that.
It consists of a photocathode, dynodes and an anode. When an x-ray photon hits the photosensitive surface it emits an electron through the photoelectric effect. This electron is accelerated towards the first dynode by a high voltage. When the electrons hit the first dynode it releases several secondary electrons. This electron cascade continues until the electrons hit an anode where they are collected and create an electrical signal.
