Ionization Chambers III

1
Ionization Chambers III

• Charge and Current Measurements

2
General Considerations

• The typical order of magnitude of charge or
  current to be measured from ionization chambers
  can be estimated from the fact that an exposure
  of 1 R generates a charge of ?3 ? 10-10 C in 1
  cm3 of room-temperature air at a pressure of 1
  atm
• In most practical cases, ion currents lie in the
  range 10-6 to 10-14 A
• The measurements of such small currents, or the
  corresponding charges integrated over usual
  irradiation time intervals of seconds or minutes,
  requires careful technique and appropriate
  instrumentation

3
Electrometers

• Electrometers can be thought of simply as
  ultrahigh-impedance voltmeters
• The gold-leaf electroscope, when equipped with a
  quantitative scale, qualifies as an early
  electrometer
• Quartz-fiber electrometers are still in limited
  use with the Victoreen r-meter and in
  self-reading pocket ion chambers

4
Electrometers (cont.)

• Most modern electrometers are of the solid-state
  operational amplifier type, which is entirely
  satisfactory for practically all ion-chamber
  applications
• Such electrometers provide a variety of ranges of
  charge measurement by means of several built-in
  input capacitors, and several current ranges with
  build-in resistors

5
Electrometers (cont.)

• For maximum flexibility of application it will be
  advantageous if the electrometer is designed with
  an inner chassis (and low-impedance terminal)
  that can be either grounded or biased at high
  voltage relative to the grounded case, as shown
  schematically in (a) and (b) in the following
  diagram
• The front panel should also be equipped with both
  a coaxial and a triaxial terminal, or a triaxial
  terminal and a coaxial adapter as in (c)

6
(a) Coaxial-cable hookup with HV on chamber
wall, and collector and guard ring at ground
potential. (b) Triaxial-cable hookup with HV on
chamber collector and guard ring, and chamber
wall grounded. (c) Adapter from triaxial
terminal to coaxial cable. Note that the outer
conductor of the triaxial terminal is not used.
7
Electrometers (cont.)

• A separate adjustable dual-polarity HV source can
  then be used to bias either the ion-chamber
  collector or the chamber wall at any selected
  voltage and polarity relative to the other, as
  indicated in the figure
• Applying a given negative voltage to the chamber
  wall with the collector at ground potential is
  equivalent to applying an equal positive voltage
  to the collector with the wall at ground
• The guard ring remains at or near the same
  potential as the collector in either case, which
  is desirable to minimize leakage

8
Electrometers (cont.)

• Some electrometers have a built-in battery power
  supply to bias the internal chassis w.r.t. the
  case, but this convenience is outweighed by the
  loss of such options as being able to check for
  ionic recombination
• Such internal batteries also tend to be forgotten
  as they gradually lose their voltage and allow
  more and more recombination to occur, and may
  eventually damage the electrometer by corrosion

9
High-Voltage Supplies

• The HV power supply for biasing the ion chamber
  should be capable of providing, with front-panel
  controls, potentials from 0 to 500 V for cavity
  chambers, or 0 to 5000 V for free-air chambers
• Good regulation against line-voltage fluctuations
  is essential, since HV fluctuations induce
  current to flow in the electrometer input circuit

10
High-Voltage Supplies (cont.)

• A free-air chamber having a voltage divider to
  bias the field-guarding wires requires sufficient
  output current for that purpose
• Otherwise the HV supply should be equipped with a
  series resistor (107 ?) inside the cabinet to
  limit the output current for safety
• Measuring the output voltage then would require a
  voltmeter having an input impedance of at least
  109 ?

11
High-Voltage Supplies (cont.)

• If a continuously adjustable HV supply is not
  available, a factor-of-two voltage change should
  be provided for assessing the degree of ionic
  recombination in the chamber
• A battery pack may be used for economy, but
  should be fully enclosed, output-limited for
  safety, and monitored regularly to verify its
  output voltage

12
General Operating Precautions

• The electrometer input-shorting switch should be
  routinely kept closed except when a measurement
  is underway, to ensure that it will not be
  unintentionally left open during any action that
  might create electrical transients, such as
  turning on the electrometer, connecting the HV,
  or changing the input cable connections. Leaving
  the switch open also risks running the
  electrometer off scale due to excess
  charge-collection. Instability or even damage
  may result in such cases.

13
General Operating Precautions (cont.)

• All parts of the electrical circuit connected to
  the electrometer input must be well insulated and
  electrostatically shielded, for example by the
  use of coaxial or triaxial cables. Inadequate
  insulation results in electrical-charge leakage
  into or out of the system, observable as positive
  or negative background current when radiation is
  absent. Inadequate electrostatic shielding is
  apparent through sensitivity of the system to
  motion of nearby objects, such as a hand waved
  around the chamber or cable.

14
General Operating Precautions (cont.)

• All input-circuit cables should be of the
  non-microphonic type, and should not be kinked,
  twisted, stepped on or flexed. Rough handling
  can cause large and variable background currents
  that may persist for hours. Cable-connector
  insulators should not be touched, blown into with
  the breath, or allowed to get wet or dirty.
• A single common electrical ground should be
  connected to all equipment cases and cable shields

15
Charge Measurement

• The classical electrometer circuit for measuring
  charge by a null method (i.e., using the
  electrometer only as a null-detecting voltmeter)
  is shown in (a) in the following figure
• Before discussing the operating procedure the
  capacitor C and the standard potentiometer will
  be described

16
(a) Classical null method for measurement of
charge with an electrometer (E). Potential P is
supplied by a standard potentiometer, S is the
input shorting switch, and C the known
capacitance upon which charge Q is collected by
the potential P, where Q CP when E is at null.
17
Capacitors

• The capacitor C upon which the ionization charge
  is to be collected should be a high quality
  three-terminal polystyrene-dielectric type, that
  is, with a groundable metal case not connected to
  either lead
• To cover the usual charge ranges it will be
  convenient to obtain at least four such
  capacitors (10-11, 10-9, 10-7, and 10-5 F) and
  preferably the three intermediate decades also
• These should be mounted inside (and grounded to)
  a metal box with the leads exiting through
  separate BNC connectors

18
Capacitors (cont.)

• The absolute values of such capacitors can be
  calibrated within 0.1 by means of a 1000-Hz
  precision AC capacitance bridge, the calibration
  of which should in turn be traceable to a
  standards laboratory
• It is assumed in this procedure that the
  capacitors have nonlossy dielectric, thus
  having the same capacitance value for DC
  electrometer use as that measured by the AC bridge

19
Standard Potentiometers

• To measure charge by the null method shown in the
  diagram, not only must the value of capacitance C
  be known, but the potential P applied to it as
  well
• A standard potentiometer can deliver known
  potentials in the ranges 0 to 0.2 V and 0 to 2.0
  V, internally calibrated against a standard cell
• The accuracy of the dial and slidewire settings
  should be verified at a standards laboratory, but
  are generally found to be closer than 0.1 of the
  true potentials

20
Standard Potentiometers (cont.)

• Any highly stable and accurate, continuously
  adjustable voltage supply (e.g., Zener-diode
  stabilized) may be substituted, noting that
  higher voltages may be used with reciprocally
  lower capacitors C to measure a given range of
  charge values, since Q CP

21
Operating Procedure

• The sequence of steps involved in a charge
  measurement by the circuit illustrated in (a) is
  as follows
• Select the value of C and the potentiometer
  range to accommodate the charge to be measured.
• Make a trial irradiation to select the most
  appropriate electrometer voltage-sensitivity
  scale, that is, where the needle takes at least
  several seconds to reach full scale, and the
  potentiometer can easily be manually adjusted to
  keep the needle near the null position (for
  conventional free-air chamber measurements) or at
  least continuously on scale.

22
Operating Procedure (cont.)

• With switch S closed, adjust the electrometer
  zeroing control to set the needle at an arbitrary
  null reading, which need not be zero midscale
  may be more convenient. The input circuit is now
  at ground potential, and will be again whenever
  the electrometer indicates a null reading. Set P
  at zero.

23
Operating Procedure (cont.)

• With the radiation shutter closed, open switch S,
  isolating the input circuit at high impedance
  from ground. A slight offset from the
  electrometer null may be observed due to charge
  separation by contact potentials in the switch
  contacts. If so, readjust the electrometer zero
  to again read exactly null with S open. The
  input is now at ground potential, although
  insulated from ground.
• Open the shutter to begin a timed irradiation of
  the ion chamber. Adjust P continuously to keep
  the needle on scale.

24
Operating Procedure (cont.)

• Immediately after the shutter closes, fine-adjust
  P to give an exact null reading on the
  electrometer. If the collected charge is
  positive, this will require a negative potential
  P to be applied to the lower side of C. Since
  the upper side of C must be at ground potential,
  the voltage across C is therefore equal to P, and
  the positive charge stored in C is Q CP.
• The background charge should be measured in the
  same manner for approximately the same elapsed
  time with the radiation source turned off.

25
Racetrack Timing

• The following variation of the preceding method
  allows measurement of charge collected in a
  measured time interval without use of a shutter.
• With S closed, adjust the electrometer zeroing
  control to set the needle somewhat below the
  midscale point. Midscale will be taken as the
  null point. Set P at zero.
• With the radiation beam already turned on, open
  S. As the needle crosses the midscale, start the
  stopwatch or timer. Adjust P to keep the needle
  on scale.

26
Racetrack Timing (cont.)

• When the irradiation is long enough for desired
  timing accuracy, over-adjust P to move the needle
  below the midpoint. Then stop the timer as the
  needle again passes the midpoint. As in the
  preceding method, Q CP. Although the input
  circuit is slightly offset from ground potential
  when the needle is at the midpoint null position,
  the offset is the same at the start and finish,
  so Q is still the total charge collected during
  the timed interval, none being trapped on the
  distributed capacitance.

27
Automatic Feedback Operation

• Clearly it would be more convenient, and probably
  more accurate, to arrange for the potentiometer
  to be automatically adjusted to make the
  electrometer reading remain constantly at its
  null setting
• With modern high-gain electrometers this can be
  done very simply by means of a negative-feedback
  loop that also eliminates the need for the
  external potentiometer in the circuit, as
  illustrated in (a) in the following diagram

28
(a) Operational-amplifier electrometer circuit
for charge measurement
29
Automatic Feedback Operation (cont.)

• The open-loop gain G of the operational amplifier
  may be taken typically as 105, meaning that if a
  small negative potential Pi is applied to the
  negative (inverting) input terminal (the
  positive input terminal being grounded as shown),
  a positive potential of P0 105 Pi will
  simultaneously appear at the output terminal
• Thus the potential across the capacitor C (with
  shorting switch S open) is Pi P0, with the
  indicated polarity

30
Automatic Feedback Operation (cont.)

• Consider what happens when negative charge Q
  flows from the ion chamber
• The input circuit is driven to a negative
  potential Pi, but as it does so the output
  potential rises to a 105 times greater positive
  potential, P0, which is applied to capacitor C
• The total potential across C is then P0 Pi, and
  it holds a charge C(P0 Pi) Q CiPi, where Ci
  is the distributed capacitance of the input
  circuit to ground
• The input impedance of the operational amplifier
  may be assumed to be too high to allow the
  passage of any significant charge

31
Automatic Feedback Operation (cont.)

• Because C is now a built-in capacitor in the
  electrometer, it should be calibrated in situ
• There are usually several capacitors of different
  values that can be switched into the circuit to
  change ranges on modern electrometers, and each
  needs calibration
• The output voltmeter is usually readable directly
  on a scale that is calibrated in coulombs in
  place of volts when charge is being measured

32
Automatic Feedback Operation (cont.)

• The method of calibration basically is to inject
  a known charge into the electrometers input
  terminal, and compare it with the resulting
  charge reading on the electrometer
• This can be done accurately and easily for
  several points on each charge scale by the method
  shown in the following diagram, in which Cs is a
  high-quality capacitor of known value

33
Calibration method for a charge-measuring
feedback-controlled electrometer
34
Automatic Feedback Operation (cont.)

• The following steps should be followed
• With S shorted and P set at zero, adjust the
  electrometer scale or digital display to zero on
  the charge range to be calibrated.
• Open S rezero as necessary. Adjust P from zero
  in stepwise fashion, for example, to 0.1 V, 0.2
  V, which will insert charges of 0.1Cs, 0.2Cs,
  into the input circuit. The electrometer charge
  readings, if correct, should be identical to
  these values.

35
Automatic Feedback Operation (cont.)

• Return the P-setting to zero to check that the
  electrometer again indicates zero, and hence that
  no charge has leaked on or off of the input
  circuit during the preceding sequence.
• Close S, set the electrometer to the next charge
  range, and repeat steps 1 through 4, until all
  ranges have been calibrated.
• Such a calibration should be repeated
  periodically (say, yearly), although significant
  capacitance changes are uncommon

36
Current Measurement

• The classical electrometer circuit for measuring
  current by a null method (i.e., using the
  electrometer as a null-detecting voltmeter) is
  shown in (b) in the following diagram
• Before discussing the operating procedure, the
  resistor R will be described

37
(b) Classical null method for measurement of
current with an electrometer. Known high-megohm
resistor R replaces capacitor C in the circuit.
The ionization current I passes through R, thus
generating a potential drop IR that is equal to
the potential P when E is at null.
38
High-Megohm Resistors

• The resistor R must of course be large compared
  to the internal resistance of the series
  potentiometer, and small compared to the input
  resistance of the electrometer
• Since these two limits are usually at least ten
  orders of magnitude apart, they allow R to be
  selected simply to generate a convenient IR drop
  across it when the input current I passes through

39
High-Megohm Resistors (cont.)

• Conventional carbon resistors are available in
  values up to 108 ? with higher values one must
  use so-called high-megohm resistors
• These are usually sealed in glass or plastic
  envelopes to protect them, and coated with
  silicone varnish to resist humidity-induced
  surface leakage
• The silicone must be kept clean only the metal
  leads may be handled

40
High-Megohm Resistors (cont.)

• Carbon-film-type high-megohm resistors tend to be
  unstable, electrically noisy, and to have high
  negative coefficients w.r.t. voltage and
  temperature
• Metal oxide types of high-megohm resistors are
  generally better in all these respects, but are
  presently available only up to 1011 ?
• Resistances greater than this value are not used
  in most commercial electrometer circuits

41
Operating Procedure

• The sequence of steps involved in a current
  measurement by the circuit shown in (b) is as
  follows
• Select the value of R and the range of P to
  accommodate the current being measured.
• Select the electrometer voltage-sensitivity scale
  that will provide the largest possible on-scale
  reading when the current is flowing through R,
  and P is set at zero.

42
Operating Procedure (cont.)

• With the radiation beam turned off and P set at
  zero, open the input shorting switch S and set
  the electrometers zero adjustment to give a zero
  reading, which will be taken as the null point.
  This compensates for any background current. (If
  it is desired to know the value of the background
  current, it can be observed separately by setting
  the null with S closed, then opening S and
  measuring the current with the radiation source
  turned off.)

43
Operating Procedure (cont.)

• Turn on the radiation source. For a constant
  ionization current I, a constant voltage IR will
  be developed across R, resulting in a constant
  electrometer reading. Now the potentiometer is
  to be adjusted to a value P -IR, which brings
  the electrometer reading back to its null
  position again, since the input circuit is thus
  restored to the same potential it had just before
  the radiation was turned on.

44
Automatic Feedback Operation

• As in the case of charge measurement with a
  capacitor, current measurement with a resistor
  can be done automatically by means of a feedback
  loop as in (b) in the following diagram
• With negative current I flowing from the ion
  chamber and through R, the IR drop equals Pi P0
• For an open-loop gain G 105, P0 105 Pi hence
  IR (1 10-5) P0 ? P0
• The input potential Pi remains practically equal
  to zero

45
(b) Operational-amplifier electrometer circuit
for current measurement, for which the
high-megohm resistor R replaces C
46
Automatic Feedback Operation (cont.)

• Because R is usually a built-in resistor in the
  electrometer, it requires calibration in situ
• There are usually several resistors of different
  orders of magnitude that can be switched into the
  circuit to change ranges, and each needs
  calibration
• The output voltmeter is usually readable directly
  on a scale calibrated in amperes in place of
  volts when current is being measured
• A convenient method of calibration is to flow a
  known current into the electrometer input and
  compare it with the resulting current reading of
  the electrometer

47
Calibration method for a current-measuring
feedback-controlled electrometer. The output
impedance of the current source must be large
compared to R.
48
Atmospheric Corrections Air Density

• The charge or current collected from an ion
  chamber in a given field of radiation depends on
  the mass and type of gas in the chamber
• If, as is most often the case, the chamber volume
  is open to the ambient atmosphere and is allowed
  to reach temperature equilibrium with its
  surrounding, the air density inside can be
  calculated from
• where Pw is the partial pressure of water
  vapor

49
Air Density (cont.)

• The barometric pressure P, temperature T, and
  water-vapor pressure Pw all should be measured by
  suitable instruments located in the same room as
  the ion chamber
• T should be measured to within 0.2C at a
  location near the chamber, allowing adequate time
  for temperature equilibrium to be reached after
  the chamber is placed in position
• P should be measured to within 0.5 torr
• Pw should be determined within about 1.3 torr,
  through a measurement of RH within 7

50
Air Density (cont.)

• In practical ionization measurements the presence
  of humidity in the air is often ignored because
  of the extra nuisance it involves, and because
  the effect of humidity on W/e is such that it
  works in opposition to the density change when
  correcting the observed ionization to the value
  that would result if the chamber contained dry
  air at 22C, 760 torr

51
Effect of Humidity on (W/e)air

• For dry air exposed to x-rays or other low-LET
  radiation, W/e may be taken to have the value
• (W/e)h for humid air is less, and the ratio
  (W/e)h/(W/e)a has been reported to follow the
  lower curve shown in the following diagram

52
(No Transcript)
53
Effect of Humidity (cont.)

• A decrease in W/e means more ionization is
  produced by a given expenditure of energy
• The effect is nonlinear, with the first 20 RH
  causing as much decrease in (W/e)h as the RH
  change from 20 to 90
• The upper curve in the figure shows how the
  ionization Qh produced in a B-G cavity ion
  chamber varies with the air humidity, assuming a
  constant chamber volume, temperature, and
  atmospheric pressure

54
Effect of Humidity (cont.)

• Note that the density and mass of gas in the
  chamber under these conditions decrease with
  increasing humidity
• This decrease in mass tends to reduce Qh in
  opposition to the effect of (W/e)h increasing Qh
  as the humidity increases
• Qh/Qa is fortuitously flat, having a value 1.0028
  0.0003 over the range 15-75 RH

55
Atmospheric Correction of an Exposure-Calibration
Ion Chamber

• The calibration of ion chambers in terms of x-
  and ?-ray exposure is a service provided by
  standardization laboratories
• Such calibrations are discussed generally in the
  next chapter we will only consider the
  atmospheric correction here

56
Atmospheric Correction (cont.)

• The exposure calibration factor of a chamber for
  a specified quality of x or ? radiation is given
  as
• in which X is the free-space exposure at the
  point occupied by the center of the chamber, and
  M is the charge collected from the chamber as a
  result of that exposure, normalized to 22C and
  760 torr

57
Atmospheric Correction (cont.)

• M is normalized to 760 torr and 22C by the
  calibrating laboratory through application of the
  equation
• where M is the charge measured under the
  existing calibration conditions, and M is the
  corrected value to be divided into the exposure X
  to give the calibration factor NX

58
Atmospheric Correction (cont.)

• Since such a value of NX is correct for typical
  laboratory humidity conditions, no humidity
  correction should be applied in using it
• In the event that a standardization laboratory is
  known to correct for atmospheric humidity in
  evaluating the factor NX, then the user should do
  so also

59
Relationship of Ionization to Absorbed Dose in an
Ion Chamber

• The ionization Q produced in any gas is related
  to the absorbed dose D in the gas by
• where each quantity refers to the gas under
  the actual conditions of the measurement
• If humid air occupies the chamber, then Q is the
  charge produced in the chamber, and ? is the
  density of the humid air

60
Ionization to Absorbed Dose (cont.)

• V may usually be assumed to be independent of
  humidity but for some wall materials (Nylon,
  A150 plastic) storage under humid conditions
  causes swelling
• V is not an immediate function of the ambient
  humidity at the time of the measurement, however
• W/e is the value appropriate for the air at the
  existing humidity level
• D is the corresponding absorbed dose in the humid
  air