Difference between revisions of "Jie's Introduction"

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There is a need for a new and innovative photon detector that can replace the photomultiplier tube (PMT) in particle-physics experiments. This new photon detector has to fix the shortcomings of the current photon detector, yet retain the advantages that the PMT provided. If such a photon detector could be found, it would revolutionize the way photons are detected in high energy physics and could influence the design and structure of detectors designed for High Energy Physics.  
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The kinetic theory explains temperature as the collective effect of the motion of many particles. Usually these collective effects are only observed as the average behavior of millions of billions of particles, which all share a common pool of energy. According to kinetic theory, all of the particles which share a common pool of energy are called members of an ensemble. Each member is free to use a random amount of energy from the shared pool, but one particle using a lot of energy leaves less energy for the other particles. This means that the majority of the particles in an ensemble have energies close to or less than the average energy, while a few of them have energies much larger than the average. When the energy distribution of the ensemble reaches a steady state, the ensemble is said to be in thermal equilibrium. According to this kinetic theory, the average energy per particle for an ensemble in equilibrium is called temperature. The energy distribution of the members of an ensemble in thermal equilibrium at temperature T is an exponential distribution with an average energy kT, where k (Boltzmann's constant) is there in order to convert temperature from degrees Kelvin to units of energy (Joules).  
  
This new photon detector, the Silicon Photomultiplier (SiPM) attempts to fix the primary flaws of the PMT, namely its large size and its sensitivity to magnetic fields with a completely different method of detecting photons.  
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According to the exponential distribution, very few particles have a large amount of kinetic energy, but no matter how high the energy or how low the temperature, the population is never quite zero. This means that even processes that require very large amounts of energy will take place in a system in thermal equilibrium at any temperature, given enough time. An interesting test of this theory would be to set up an experiment to look for those rare instances when an ensemble contains a particle with energy many times the average given by the temperature. This experiment has been carried out using a novel detector comprised of a large array of silicon avalanche photodiodes known as a silicon photomultiplier (SiPM). The avalanche photodiode works like a mousetrap, storing a large amount of energy and then releasing it suddenly in response to a weak disturbance. In its intended mode of operation, the weak disturbance is provided by the absorption of a single photon of visible light in the region of the diode junction. In this experiment, the device was shielded from all external light sources, so that the only possible trigger mechanism is the internal motion of electrons within the junction itself. According to the kinetic theory, even without photons to excite the electrons over the trigger threshold, from time to time an electron should acquire enough energy to simulate an absorbing photon just from the randomness of the thermal energy distribution. The rate at which these thermal triggers occur is predicted by the kinetic theory, based on the exponential distribution, the temperature of the junction, and the number of electrons in the region of the junction. This mechanism reacts to the energy of a single electron, allowing us to detect the thermal energies of a single particle.  
  
[Insert 2 pictures comparing the two different methods of detecting photons]  
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Instead of using a series of dynodes to amplify the signal of the photon, the SiPM uses 2 highly charged electron plates that are placed closely together. When a photon lands on one of the plates, the energy in the photon dislodges one of the electrons on the charge plate which releases the energy built up in the plates, causing a pulse. This method of detecting photons makes it impervious to the strongest of magnetic fields because the entire process happens electrically. The PMT, on the other hand, needs the electron to travel a certain trajectory, therefore making it very susceptible to magnetic fields. If the electrons in the PMT veer off from their course due to magnetic fields, then they won't make it to the next dynode and produce a pulse. Also, since the inner workings of the SiPM are all electrical, it is possible to be made much smaller in size compared to the PMT.
 
 
 
With these two primary advantages of the SiPM over the PMT, the SiPM seems to be a suitable replacement as the leading photon detector in particle physics.
 
 
 
While the SiPM has these advantages over the PMT, physicists must also be sure that it can perform acceptably in situations where the PMT excels. One such attribute is the PMT's resistance to temperature. It shows the same excellent performance at almost any temperature as long as the temperature isn't as high as its melting poing. In contrast, the semi-conducting material used to make the SiPM depends heavily on the temperature that it operates at. A higher temperature will make the SiPM much more conductive, and therefore, more likely to produce a pulse, and in consequence, may cause many more false detections. The rate of these false detections, when occur due to the presence of thermal energy in the SiPM, is called Dark Rate.
 
 
 
The effect temperature on the Dark Rate of the SiPM must be examined to determine the range of temperatures that the SiPM can opperate effectively.
 

Latest revision as of 19:57, 31 January 2008

The kinetic theory explains temperature as the collective effect of the motion of many particles. Usually these collective effects are only observed as the average behavior of millions of billions of particles, which all share a common pool of energy. According to kinetic theory, all of the particles which share a common pool of energy are called members of an ensemble. Each member is free to use a random amount of energy from the shared pool, but one particle using a lot of energy leaves less energy for the other particles. This means that the majority of the particles in an ensemble have energies close to or less than the average energy, while a few of them have energies much larger than the average. When the energy distribution of the ensemble reaches a steady state, the ensemble is said to be in thermal equilibrium. According to this kinetic theory, the average energy per particle for an ensemble in equilibrium is called temperature. The energy distribution of the members of an ensemble in thermal equilibrium at temperature T is an exponential distribution with an average energy kT, where k (Boltzmann's constant) is there in order to convert temperature from degrees Kelvin to units of energy (Joules).

According to the exponential distribution, very few particles have a large amount of kinetic energy, but no matter how high the energy or how low the temperature, the population is never quite zero. This means that even processes that require very large amounts of energy will take place in a system in thermal equilibrium at any temperature, given enough time. An interesting test of this theory would be to set up an experiment to look for those rare instances when an ensemble contains a particle with energy many times the average given by the temperature. This experiment has been carried out using a novel detector comprised of a large array of silicon avalanche photodiodes known as a silicon photomultiplier (SiPM). The avalanche photodiode works like a mousetrap, storing a large amount of energy and then releasing it suddenly in response to a weak disturbance. In its intended mode of operation, the weak disturbance is provided by the absorption of a single photon of visible light in the region of the diode junction. In this experiment, the device was shielded from all external light sources, so that the only possible trigger mechanism is the internal motion of electrons within the junction itself. According to the kinetic theory, even without photons to excite the electrons over the trigger threshold, from time to time an electron should acquire enough energy to simulate an absorbing photon just from the randomness of the thermal energy distribution. The rate at which these thermal triggers occur is predicted by the kinetic theory, based on the exponential distribution, the temperature of the junction, and the number of electrons in the region of the junction. This mechanism reacts to the energy of a single electron, allowing us to detect the thermal energies of a single particle.

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