Detectors Exam 2

Where does the saturated current occur on a graph?

the y-intercept. (isat = 1/y-int)

How does the ion current vs wall thickness plot look and trend?

The plot increasing quickly until it begins to stop and stay constant. As the wall thickness increases the ion current will also increased until all radiation is attenuated in the wall and the current can not change. 

What are the trends of ion current vs wall thickness in a vacuum?

If the chamber is surrounded by a vacuum, there’s no surrounding air to produce extra secondary electrons that would normally enter the chamber through the wall. The loss of those “compensating” electrons from the air makes the chamber under-respond (it shows less ion current than it should).

Why is air widely used as a fill gas for ionization chambers but seldom used in proportional counters?

Oxygen has a large electron attachment coefficient, and when oxygen attaches electron they turn into negative ions and move much slower and would be unlikely to reach the small multiplication volume around the anode wire to be a good proportional counters. On the other hand air is useful in ionization chambers since electrons or the negative ion formed when the electron becomes attached, can be collected. An air-filled chamber also provides a direct reading or exposure.

Describe a measurement that can determine whether a proportional counter tube is operating in the proportional region.

Using different but known energy depositions and measure the resulting pulse amplitudes. If the ratio of the pulse amplitudes is consistent with the ratio of energy depositions, the tube is operating in proportional mode.

In a windowless proportional counter, the output pulse height from an alpha particle source will increase with increasing alpha energy, whereas for the beta particles, the opposite is usually true.

For alpha particles, there range is short and deposit all their energy at one point. If incident energy increases so does deposited energy, so the pulse height increases as well. For beta particles, the range is long, usually longer than the tube. As the incident energy increases, the deposited energy decreases and hence the number of the carriers and pulse height also decreases, due to less interactions at higher energy.

Explain the function of the “field tube” found in some proportional tube designs. 

The function of the field tube is to reduce the electric field near the ends of the tube where the distortion in the electric field would occur. 

Why are there distortions in the electric field near the ends of the tube in a proportional counter?

The cylindrical geometry allows for no distortions; however, towards the end of the tube, the anode is no longer perfectly cylindrical, which ruins the geometry. ,

Explain the sudden discontinuities in the curves for the absorption properties of various gases shown in Fig 6.18.

These discontinuities are absorption edges based on the shell. K is the innermost shell (tightest) with the highest binding energy. When the particle approaches the binding energy of the shell, the probability of the PE occurring increases. Once it passes that shell, the probability of PE decreases dramatically till it is near the next shell.

Why are gas scintillation proportional detectors normally operated with a voltage below that required for charge multiplication?

In gas scintillation detectors, photons from excited gas atoms form the signal. Operating in the proportional region increases photon yield but introduces random fluctuations from gas multiplication, degrading energy resolution. Therefore, these detectors are run just below the multiplication threshold, where excitation (and photon emission) occurs without avalanche ionization to preserve good energy resolution.

Why must the quench gas in a GM tube have an ionization potential below that of a major fill gas?

The quench gas acts like a “shock absorber” — it takes the charge from the main gas ions so those ions don’t hit the cathode and restart another discharge, which would cause false events.

The quench gas takes over their charge before they reach the cathode. Because the quench gas has a lower ionization potential, it’s easier for it to give up an electron and take the charge.

How does doubling the diameter of the anode wire effect the starting voltage for a GM tube?

The number of multiplication will decrease which means voltage must increase to compensate.

How does doubling the fill gas pressure effect the starting voltage for a GM tube?

The multiplication will decrease slightly so voltage will have to increase slightly.

How does doubling the trace concentration of the quench gas effect the starting voltage of a GM tube?

Increases the concentration of the quench gas ill increase the value of k and delta V which will decrease the number of multiplication which means the voltage needs to increase slightly. 

Why does the pulse height from a GM tube continue to increase with applied voltage even after fill Geiger discharge is obtained?

• The GM pulse stops growing because positive ions build up around the wire and weaken the field (space charge effect).

• A higher voltage makes the field stronger, so it takes longer for the ions to weaken it → the pulse becomes larger.

• Eventually, this still stops automatically — that’s why the GM tube gives one big pulse per event, regardless of energy.

How do GM tube and proportional counters differ in behavior with regard to variation of pulse height with applied voltage?

Proportional: The pulse height varies as the avalanche amplitude varies, which depends on the voltage in an exponential manner.

Geiger: The pulse amplitude corresponds to the number of ion pairs at which the accumulated positive space charge is sufficient to reduce the electric field below its critical value. Also increases in proportion to the og electric field or linearly with the applied voltage.

How do GM tube and proportional counters differ in behavior with regard to the need for a quench gas and its function?

Proportional: The quench gas must absorb UV photons.

Geiger: The quench gas must pick up positive charges from the original positive ions through charge transfer collisions.

How do GM tubes and proportional counters differ in behavior with regard to their ability to differentiate heavy charged particles and electron radiations?

Proportional: Heavy charged particles deposit all of their energy in one spot, but electrons lose a little all over the place, so they will have different pulse heights.

Geiger: No differentiation because pulse height is independent of particle type and energy.

How do GM tube and proportional counters differ in behavior with regard to their ability to register high counting rates?

Proportional: The maximum counting rate is often set by pulse pile-up. The minimum pulse shaping time is limited by the finite rise time of pulses

Geiger: The maximum counting rate is determined by the long dead time of the tube itself.

How do GM tubes and proportional counters differ in behavior with regard to typical counting efficiency for 1 MeV gamma rays?

Proportional: Gamma rays produce very small amplitude pulses and are often below the discrimination level.

Geiger: Counting efficiency is a few percent due primarily to the liberation of secondary electrons from the detector walls.

The intrinsic detection efficiency for a gas-filled counter when used with medium-energy gamma rays will depend on the counter wall thickness. Explain the graph.

For gamma rays the tube depends on the wall to induce (y,e-) reactions, The reactions causes the electrons to move through gas and create ion pairs that lead to avalanching. The greater the wall thickness the greater probability a reaction occurs and will lead to an avalanche but efficiency decreases because the change the electrons go through the wall decrease.

How do you determine an approximate wall thickness?

The range of a q MeV electron is 1-2 mm/ Mev so anything thicker would be pointless because the particle coudnt’t make it out of the wall.

A GM tube is operated with a counting system whose threshold requires that a full Geiger discharge must take place to register a count. Is it dead time behavior likely to be better described by the paralyzable or non-paralyzable model?

Paralyzable. After an avalanche, the positive ions around the anode must be cleared to the cathode to reestablish the full electric field in the tube. As the positive ions move to the cathode, another reaction creates a smaller avalanche and requires the clearing process to occur again. The extendable dead time proves paralyzable.

Is an inorganic scintillator or organic scintillator better for speed of response?

Organic are faster than inorganic.

Is an inorganic scintillator or organic scintillator better for light output?q

Inorganic are much brighter than organic.

Is an inorganic scintillator or organic scintillator better for linearity of light with deposited energy?

Inorganic tend to have a more linear response to the amount of energy deposited.

Is an inorganic scintillator or organic scintillator better for detection efficiency for high energy gamma rays?

Higher for Higher Z detectors that are associated with inorganic materials.

Is an inorganic scintillator or organic scintillator better for cost?

Plastic or liquid scintillators are generally much cheaper than inorganic bc they dont have to be grown as a single crystal. 

Explain the function of the activator added in trace quantities to many inorganic scintillators. Why are they not needed in organic scintillators?

The activator in an inorganic scintillator is the atom impurity that actually produces the detectable scintillation light. Its energy structure lies in the forbidden energy gap of the host material. When an electron is excited to the conduction band in the host material, it migrates to an activator site and non-radiatively loses energy so that it occupies the lowest level of the activators excited state. The subsequent de-excitation is at a wavelength that is too long to cause ionization in the bulk material and this scintillation photon flows freely to photodetector for detection.

Explain the following statement: Organic crystalline scintillators remain good energy to light converters when dissolved in a solvent, while inorganics no longer function as scintillators if dissolved.

An inorganic scintillator, a crystal lattice is required to establish the energy levels and the activator is chosen to have energy levels that lie within the forbidden energy gap of the host material. Without the crystal structure present, there would be no scintillation possible.