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Users Guide
 

Ion Chamber Manual

last modified 13-09-2006 10:40

This part contains information on how to optimise the detectors used at BM29 at the moment for the measurement of the beam intensity before and after the sample(s). They consist of 30cm long ion chambers with a high voltage power supply and associated electronics. This section only describes how to change the gases in the ion chambers, the adjusting of the electrometers are up to now only described in A standard experiment.

Principle

For X-ray absorption spectroscopy in transmission geometry, ion chambers are used in general at BM29 to monitor the incident and transmitted beam intensity.
The incoming x-ray beam is partially absorpt in the chamber and ionizes the gas. The produced charges are collected by the electrodes and the resulting current is measured using low noise amplifiers. For more details, refer for example to X-RAY DATA BOOKLET, Section 4.A and references therein. For optimal preformance at a given energy, the optimization of the gas filling is required. This consist of selecting an appropriate gas mixture and pressure so as to provide a suitable ionization medium over the range of X-ray energies to be studied. It is usually sufficient to optimize absorption of the I0 and I1 (incident and transmitted beam) ion chambers to approximate values of 30% and 70% in the middle of the working spectral energy range. This give optimal signal to noise ratio for standard samples with optimised thickness in transmission geometry.

  1. Optimal gas filling
  2. Calculation of the optimum gas filling
  3. Fill the detectors
  4. High voltage supply
  5. The vacuum pump
  6. The filling station
  7. The ion chamber
  1. Optimal gas filling
    We provide N2 (dry nitrogen), Ar, Kr, Xe as detection gases and He to fill up the chambers to 2 bar in order to avoid discarges in the detector at rather low filling pressure of these gases. For low energy operation, 4.5kev to 10keV, the preferred ionization gas is nitrogen. In the intermediate energy range, 10keV to 30keV, argon is more suitable, whilst above 30keV, krypton gas is utilized.

    We prefer to use the same detection gas in both (if using I0, I1 chambers) detectors in order to minimise non-linearity effects. But you are of course free to use a different strategy. For some energy regions it is indeed preferable to use mixtures, as the pressure in the detectors is limited to 2.5 bars maximum. For a given gas, this maximum pressure may not be sufficient to achieve the desired absorption in I1, while the same absorption corresponds to very low pressure for next heavier gas. As an example, if you want to optimize at 8 keV with 30% and 70% absorption in I0 and I1, respectively, you would need 4.2 bar of nitrogen in I1 (0.19 bar Ar), while for Ar filling with 30% absorption in I0, only 0.06 bar Ar is required. In general, a higher filling pressure gives better noise figures than an optimized filling with a very low pressure.

  2. Calculation of the optimum gas filling
    To facilitate simple calculation of the pressures required for BM29 ion chamber fillings, a program exists on the station computer (tolomeo). To use it, please type pression in any xterm or similar window logged in on zapata and follow the instructions.
    The program enables you as well to calculate the absorption of a gas at a given pressure and energy. If you want for example to cover a large energy range (e.g. several edges), you can calculate the optimal gas filling in the middle of the energy range and check after with this feature if the absorption is not to high or low at the extremities of the energy range.
    The following example illustrates how to calculate the pressure needed using Ar at 20 keV: 
        
	 tolomeo:~ % pression


	 ##############################################

	 ION CHAMBER SET-UP
	  1- optimal gas pressure in a chamber at a given energy
	  2- beam absorption for a given gas pressure in a chamber
	  3- exit
	 Type your choice: 1

	 **********************************************************

	 Gas available on BM29 for ionization chambers

	  1- He
	  2- N2
	  3- Ar
	  4- Kr
	  5- Xe

	 Type the number corresponding to the gas choosen (type 0 to exit): 3


	 Type the working energy (eV): 20000
	 The interpolated cross-section at 20000.00 eV is 8.669369 gr/cm^2


	 Type the absorption as fraction of unity
	 (typically 0.3 for I0 or 0.7 for I1): 0.3

	 gas type 3
	 filling for beam absorption of 30.0% at 20000.00 eV is: 0.77 bar
	 with this filling the chamber absorbs 34.17% of the beam at 19000.00 eV
                                               26.75% of the beam at 21000.00 eV
         **********************************************************

    Other possibilities to estimate the gas filling and pressure to use are to looked up the tables/figures at the beamline, work it out using the cross-sections found for example in the XOP program installed on the beamline PC (you can use this program actually to calculate the cross-section or absorption of any material, thus for example use it as well to optimise you sample thickness and experimental conditions), or have a look at X-RAY DATA BOOKLET, Section 4.A.

  3. Fill the detectors
    These are the basic steps to change the chamber filling. Details can be found below.
    1. Turn off high voltage supplies at the instrument control crate
    2. evacuate the gas lines and chambers
    3. fill the detectors with the desired pressure.
    4. Turn on the high voltage supplies, switch off the vacuum pump

    Comments:
    • If you fill several chamber at the same time (e.g.) I0, I1, it is convenient to pump all chambers together, then put the detection gas sequentially for all chambers. Pump the filling line, then fill up to 2 bar with He, starting with the chamber of the lowest pressure (in general I0).
    • For pressures from some 0.3 bar on, you put first the detection gas at the desired pressure (for all chambers to fill), then pump the filling line, and fill up the chambers with He to a total pressure of 2 bar.
      If you wish to use lower pressures of the detection gas, it is preferable to first fill the chamber with He to some 2 bar minus the choosen pressure of the detection gas, and then add the detection gas. The pressure of the choosen gas should of course be higher than 2 bar (default is ~ 2.2 bar anyway).

  4. High voltage supply
    The high voltage (HV) supplies are situated on the lower part of the bench, on the right side of the crystat table and labeled I0, I1, I2 for the correspondent ion chamber.

    To turn off the correspondant HV for the detectors before pumping ! To do so, just switch the HV off (on/off switch). Switch HV back on after you finished changing the gas filling of the detector.
    We normally work at 1000V, you do not need to adjust the voltage.


    Fig. 6.1: HV supplies.
    Watch out:
    • If you pump on the chambers with the HV on, you risk discharge that will damage the detector. Nearly for sure the power supply will go on failure, and it is likely that you damage it.

  5. The vacuum pump
    The vacuum pump is placed in front of the HV rack. The right valve (Fig. 6.1, valve p2) opens to the filling circuit, the left one (Fig. 6.1, valve p1) is used for the cryostat evacuation. Don't use them in parallel.

    To use the pump for pumping the ion chambers, switch on the pump, close the left valve (if not already done) and open the right valve.

    Don't forget to switch off the pump after use.

  6. The filling station
    The filling station is mounted on the wall behind the bench (ring side). There are 4 gas lines for bottles stored outside of the hutch, plus a line for the use of dry nitrogen. At the moment the gases available are He, N2 (dry nitrogen), Ar and Kr. Each of the principle 4 gas lines is equipped with a valve to isolate the gas bottle (Fig. 6.2, valve f1), a pressure regulator with pressure gauge (you normally should not need to change the pressure of the gas !) and a valve (Fig. 6.2, f2) to open to the filling lines of the ion chambers.

    The 4 top valves f2 are closed in the horizontal position and open in vertical position (i.e. the direction of the tube). Watch out that the green f2 valve for N2 is open in horizontal and closed in vertical position ! You only open the correspondent valve f2 when you want to fill the chamber and close it immediately after. On Fig. 6.2, all valves f2 are closed for He, Ar, Kr, and 'free', while N2 is open. All valves f1 on Fig. 6.2 are closed.

    Pressure gauge on the filling line
    The additional large gauge show the pressure in the filling line. The gauge shows the pressure relative to normal pressure, that is that for a pumped filling line the gauge should be at -1 bar. On Fig. 6.2, the absolut pressure of N2 in the filling line is therefore 2.7 bar.

    Pump the filling line
    To use a gas, pump first the filling line: Check that all valves f2 are closed and the pump is running. Open valve f3 and wait for the pressure to go down.

    Flush the filling line
    You may flush the lines 2-3 times, specially if you change from a heavy gas to a lighter one. To do so, pump as described, then close f3. Open the valve f1 corresponding to the gas you want to use and then open slowly the corresponding valve f2 until the gauge shows around '0' in the filling line.
    Close f2, pump via f3, and repeat, if desired.

    He line for the cryostat
    The blue pipe departing above the He valve f2 is the line used to fill the cryostat with He gas. To do so, open He valve f1, while valve f2 stays close, and follow the description in the cryostat manual.

    Please close the valve f1 and f2 once you filled the chambers.


    Fig. 5.2: filling station.

  7. The ion chamber
    All ion chamber used at BM29 are of equal length (30cm). They are equipped with 2 valves, valve A for pumping, valve B for filling, and a pressure gauge (similar to Fig. 6.2).


    Fig. 6.3: Ion chamber IC2.

    Evacuate the detector
    Switch on the pressure gauge on the ion chamber you want to pump/fill by pressing one of the 2 buttons if not already switch on. It will take 2-3 senconds before it actual value is shown. The gauge switches automatically off after some minutes.

    Make sure that the vacuum pump is running. Open valve A (Fig 6.2) to pump (valve B is closed).

    Fill the detector with the detection gas
    Close the pumping valve A. After having evacuated the filling line (see above), open sequentially valve f1 and f2 of the desired gas on the filling station. Open slowly valve B, until the pressure gauge displays the desired pressure.

    Fill the detector with He to 2 bar
    Close valve B. Close valve f2 (and f1 if not needed anymore) on the filling station. Evacuate the filling line like described in the section above. Open valve f1 and f2 of He, eventually flush the filling line (specially if using heavier gases). With the He valves open, slowly open valve B on the ion chamber, until the pressure gauge displays around 2 bar.
    Comments:

    • When evacuating the ion chambers, the final pressures that can be reached with the pump in use are around 7 mbar. As a function of how clean/accurate you would like your filling, you either wait until the final pressure is reached, or flush the chamber some 2-3 times with the detection gas.

 
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