Isotope separator

• NAME
• How to do an In-111 implantation in 20 easy steps
• How to prepare an In-111 source for the isotope separator
• How to mount the ion source onto the isotope separator
• How to get a noble gas beam through the isotope separator
• Beam optics

NAME

How to do an In-111 implantation in 20 easy steps

1. Select an ion source suitable for In-111 implantations:

       plasma chamber
         length                  33 mm
         diameter                28 mm
         aperture Ø               1 mm
       quartztube
         length                  90 mm
       plug
         length                  10 mm
         outer Ø                3.5 mm
         hole Ø                   1 mm
         brim length             ~2 mm
         brim Ø                   6 mm
   

2. Outgass the source by heating it to 800-1000 °C for 30 min.
   
   
3. Mount the source onto the source holder and wrap kathode and oven filaments of tungsten wire:

       kathode filament
           wire diameter          0.5 mm
           # turns                  6
           winding diameter         4 mm
       oven filament
           wire diameter          0.5 mm
           # turns                  7
           winding diameter         7 mm
   

4. Load the ion source with InCl₃ in the B-laboratory.
   
   
5. Mount the source holder onto the separator.
   
   
6. Fix the samples and the calibration source to the target turn-table and mount the turn-table onto the target section.
   
   
7. Adjust the horizontal and vertical width of the target slits to the sample dimensions. Set beam deflection and the target sweep voltages to 0 V.
   
   
8. Check the window of the SCA. Set the lower level just above the noise and set the window to integral when the target section is not contaminated. Otherwise, set a window that includes only the 171 keV and 245 keV gamma-peaks and limit the counting rate to about 100 counts/μCi by inserting proper lead shielding.
   
   
9. Calibrate the NaI detector using the In-111 reference source.
   
   
10. Start up the computer system, the cooling system and the power supplies of the ion source.
    
    
11. Outgass the ion source by heating it to about 450 °C for 1 h.
    
    
12. Start the machine using a noble gas. Keep the power dissipation low, i.e. V_anode = 50 V and I_anode < 0.5 A.
    
    
13. Adjust the beam optical elements to In-115 (see Beam_optics).
    
    
14. Crank up the ion source to an In-115 current of about 200 nA and optimize the beam current on the dummy target.
    
    
15. Put the In-115 beam in 'Faraday cup 1' and tune the position of the Faraday cup until both the In-115 beam in the cup and the In-113 beam on 'slit 1' are optimal.
    
    
16. Optimize the In-113 beam on the dummy target and set the desired sweep.
    
    
17. Close 'valve A4', put the In-113 beam in 'Faraday cup 1', select a sample to be implanted, determine the gamma background level, start program control on de computer, and open 'valve A4' to start the implantation.
    
    
18. Crank up the ion source to an implantation rate of about 1 μCi/min. Typical ion source parameters and beam currents for an In-111 implantation are:

        anode voltage               35 V
        anode current             2.00 A
        coil current               4-6 A
        oven current           0.5-1.0 A
        In-115 current (M1)         10 μA
        In-113 current (M1)        450 nA
        In-111 current (target)     50 pA
    

19. Write down the target current regularly, in order to enable determination of the implanted doses afterwards.
    
    
20. Before implanting the next sample, write down the activity of the current sample, close 'valve A4', select a new sample, write down the background, and re-open 'valve A4'.

How to prepare an In-111 source for the isotope separator

1. Be aware of the fact that you'll be working with some fairly radioactive material (10 mCi). Wear gloves, be aware of your active and inactive hands, and don't touch clean places with your active hand. Keep all the radioactive material behind lead.
   
   
2. Open the Mallinkrodt can and take out the bottle with radioactive fluid using some gripping pliers. Remove the cap from the bottle with the aid of one of the pincers.
   
   
3. Weigh out ~13 mg InCl₃ and put it in a 5 ml beaker.
   
   
4. Add the active fluid and dry the solution using the hot plate and air blower until a few drops are left.
   
   
5. Put the drops in the quartz tube of the ion source using a pipette.
   
   
6. Wash out the beaker with 1 or 2 drops of demineralized water and put these drops also in the quartz tube.
   
   
7. Make a heat shield from aluminium foil and wrap it around the ion source, leaving only a small opening. Dry the fluid in the tube using a heat lamp and airblower but make sure that it is not going to boil. Measure the dose rate in mSv/h at a distance of about 15 cm.
   
   
8. Put the carbon plug into the quartz tube.
   
   
9. Put the source holder in the two-way cupboard.
   
   
10. Make two In-111 sources. Calibrate them using the GeLi detector set-up (HV = +2500  V). The reference source for the isotope separator should be no more then ~2 μCi (10-20% of the required sample activity) in order to prevent too high background counting rates during implantation.
    
    
11. Perform a wipe test on the floor, and check your hands and feet for contamination.
    
    You don't have to be frantic about an intake of 100 Bq or so, because it adds only about 0.0001% to your natural yearly dose.

How to mount the ion source onto the isotope separator

1. Wear gloves and take care not to contaminate the rear part (from the flange to the electrical connectors) of the source holder.
   
   
2. Mount the source holder, connect the gas inlet, close the yellow valve, and open the green valve.
   
   
3. Connect the roughing pump to the yellow valve outlet and start the pump.
   
   
4. Open the yellow valve very slowly to prevent the activity from being sucked out of the quartz tube (keep pressure reading stable at about 10 mbar).
   
   
5. Wait until the pressure has dropped to about 0.1 mbar. Close the yellow valve, turn off the roughing pump and dissconnect the pump. Close the green valve and open the main valve to the vacuum system.
   
   
6. Slide the source gently forward to its most extreme position, then slide it back 1 mm, and stick the metal spacer between the flange and one of the adjustment bolts.
   
   
7. Connect the power supply wires to the terminals. Connect the red wire from the anode (blue terminal) to the gas inlet.

How to get a noble gas beam through the isotope separator

Start the machine.

1. Switch on the source cooling and hold the green button for about 30 s to prevent the circulation pump from shutting off again on the flow detection switch.
   
   
2. Check that all settings of the power supplies for the ion sources are set to zero and then switch on the 1:1 transformer of the ion source section.
   
   
3. Check the pressure in the gas reservoir (> 100 mbar), wait a minute or so for the needle valve to close, and then open the electromagnetic valve switch of the gas reservoir.
   
   
4. Outgass a new source for 1 h at ~450 °C.

Start the source.

1. Switch on the servo control of the needle valve. Turn the dial to 4.0.
   
   
2. Set the current of the source magnet to ~2 A.
   
   
3. Turn up the filament current slowly to 16.5-17.0 A.
   
   
4. Set the anode voltage to 50 V.
   
   
5. Set the anode current to 0.5 A. Make sure that the power supply limits on the voltage. It may take about an hour before the preset current is reached. This is due to further outgassing of the chamber walls which distorts the plasma formation.
   
   
6. Keep an eye on the anode current, because it tends to rise rapidly as soon as optimum ionisation conditions have been established:

       p = 0.0001 to 0.01 mbar
       T > 600 °C
   

   In case that the power supply is set to limit the ionisation potential, you can control the anode current through the filament current (sensitive !).
   
   
7. As soon as you have a stable plasma, you can set the parameters to the optimum values (which depends on the particular isotope).

Source operation.

Two modes of source operation are available:

• fixed anode voltage
• fixed anode current

Which mode is the best depends on the temperature dependence of the plasma 'resistance' R.

ðR/ðT < 0

i.e. a higher temperature leads to a better conduction of the plasma, which is the general case. In this case it is best to fix the anode current because it ensures that ðP/ðT < 0, i.e. the power-temperature feedback is negative. To maintain a sufficiently large ionization potential (V_anode > 30 V), one should adjust from time to time the filament current.

ðR/ðT > 0

This situation only occurs when the quartz tube is filled with solid material and additional oven heating is applied. The behaviour is due to the feedback between the source temperature and the vapour pressure of the material in the oven.

If we fix the plasma current like in the previous case, we have the following chain of reactions:

    an increase of the source temperature =>
    an increase of the oven temperature   =>
    an increase of the source pressure    =>
    an increase of the plasma resistance  =>
    an increase of anode power            =>
    an increase of source temperature

For oven currents larger than 2-3 A the feedback tends to be larger than one, in which case one is forced to put more and more power into the plasma to keep it going.

Since this leads to an unstable situation, the best thing to do is to fix the anode voltage.

Getting the beam on 'focus 1'.

1. Take the ground pokers from the source section and from the power supply cabinet. Check if the soft-landing set-up is connected to the source section. Make sure that you have closed both doors (safety switches). Put 'Faraday cup 1' into the beam-line.
   
   
2. Switch on all the power supplies: high voltage, magnets, quadrupole lenses, beam profile monitors, NIM bins, position stabilizers, scope, and the pA-meter. Switch the position stabilisation to M (monitor). Switch the pA-meter to monitor 1.
   
   
3. Set the current for bending 'magnet 1' (see figure 1).
   
   
4. Turn up the HV slowly to ~50 kV. Keep an eye on the current meter of the HV power supply and the pirani of the source section (PE21) (HV discharges !).
   
   
5. Adjust the HV until the right mass peak is at the centre of the scope display.
   
   
6. Optimize the beam using 'quadrupole lens 1' (Q1) (use scoop image), 'vertical deflection 1' (V1), the current of the source coil, and the HV. Don't touch the horizontal deflection, since it is set to correct for the wringing field of bending 'magnet 1'. Approximate values for Q1 and V1 can be found in section Beam_optics.
   
   Note that for large ion beams (5-10 μA) the quadrupole voltages must be much larger. However, the voltage ratio Q1+Q2/Q3+Q4 should remain roughly the same.

Getting the beam on 'focus 2'.

1. Move 'Faraday cup 1' out and move 'monitor 2' into the beam-line.
   
   
2. Switch the pA-meter to klepje 1 and the position stabilisation to S (slit).
   
   
3. Optimize the beam on the slit by using only the HV. Check the current on the slit (secondory electron suppression M1).
   
   
4. Set the current of bending 'magnet 2' (see figure 2) and open the slit. Switch the pA-meter to monitor 2.
   
   
5. Optimize the beam by varying M2, Q2, V2, HV. The beam current should be 10-20% larger than that on 'monitor 1'. Check Q1 and V1 again, if necessary. Do not change H2, since it is directly related to H1.

Getting the beam on 'focus 4' (target section).

1. Open 'valve D7' and 'valve D4'.
   
   
2. Switch pA-meter to monitor A4.
   
   
3. Set the current of M3 (see figure 3).
   
   
4. Optimize the beam current by varying M3, V3, Q3 and Q4. The current should be approximately the same as on 'monitor 1'. If not, redo your job up to this point.

Getting the beam on the target.

1. At the target section: rotate the turn-table until the test target is in the beam, set the target slit, check that the -200 V suppressor voltage is on, set the current switch to pA-meter, and set both the sweep voltage (left-most position) and the deflection voltage (middle position) to zero.
   
   
2. Switch off 'monitor A4'. Optimize the beam current measured with the digital voltmeter at the target section by varying Q5.
   
   
3. Check that the beam is nicely centred on the target by playing around with the target deflection voltages. If not, correct this with the deflection plates V1,V2,V3,H3 or redo your job.

Beam optics

    element     dial setting    optimize

    Magn #1              435    var
    Magn #2              545    var
    Magn #3              770    var
    Hor #1               520    fix
    Hor #2              -425    fix
    Hor #3              < 50    var
    Vert #1                0    var
    Vert #2              250    var
    Vert #3               80    var
    Quad #1a             700    fix
    Quad #1b            1370    var
    Quad #2              770    var
    Quad #3             1900    fix
    Quad #4a            1850    var
    Quad #4b            1550    var
    Quad #5             1300    var
    Quad #6              850    var