1. Changing the absorbers: how does it fit in the MICE experimental programme? Besides the requirement that the amount of multiple scattering material be minimum, the physics request from MICE on the absorbers is also that of variability Which materials? Which variability? in which context and how often? let us first have a look at the running scenario

2. Consider the baseline MICE measurement: Optics = nominal optics with 3 flips (spectrometer coupling coupling and spectrometer with + - + - polarities) beta function = 42 cm, nominal momentum 200 MeV/c DE=-10.5MeV in all three absorbers and +10.5 MeV in the RF cavities on crest. to do an emittance in/out curve one needs diffuser thickness all the way from the minimum to 4 X0, probably 5 points and to cover the momentum range one needs to set the beam at steps of about one rms. (such as 150, 175, 200, 225, 250 MeV/c) This gives 25 points to measure, in principle each taking 20 minutes, (for 10-3 precision) plus time for changeovers... a good 24 hours of very busy data taking. (how long does it take to change diffuser thickness? how long does it take to change beam momentum?) (this will come after several months of debugging which can be done with plastic or whatever is most practical)

3.  Incoming muon beam Diffusers 1&2 Beam PID TOF 0 Cherenkov TOF 1 Trackers 1 & 2 measurement of emittance in and out Liquid Hydrogen absorbers 1,2,3 Downstream particle ID: TOF 2 Cherenkov Calorimeter RF cavities 1RF cavities 2 Spectrometer solenoid 1 Matching coils 1.1+1.2 Focus coils 1 Spectrometer solenoid 2 Coupling Coils 1+2 Focus coils 2 Focus coils 3 Matching coils 2.1+2.2

4. Beam possible design for absorber II remote control 4 X 0

5. a series of measurements to determine equil. emitance A determination of equil. emittance is essential in view of a muon cooling ring

6. A series of measurements

7. Now if one wants to redo this for the 6 optical configurations of the proposal (various betas and nominal momentum), this will take 6 days. Then time has come to change the absorbers (2 weeks intervention?) and to take data again for three weeks etc. It soon becomes evident that a full summer will be very usefully spent if all goes well *and* if we have the beam solenoid. §Repeat for the different polarities (solenoid mode) and one has an intense 3 weeks of data taking period with a given set of absorbers and RF volts... if all goes well.

8. The last parameter to investigate is RF volts. This can be done in several ways. -- connect the 8 MW RF power all to one 4-cavity module. This increases the local volts by sqrt(2), (and decreases the overall voltage by sqrt(2)) -- operate at L N2 temperature this allows increase of gradient and volts by 1.5 to 1.7. -- combine the two allows to reach a gradient of over 16 MV/m in one 4-cavity module obviously this is a major operation that requires lots of RF work. at the same time this will be the ONLY moment where one would need to change the thickness of the LH2 abosrbers to protect the detectors.

9. e-e-  e-e- The absorber plays a role beyond provideing dE/dx: protects trackers against RF electrons! This is helped by the magnetic field…. But only in the flip situation.

10. Preliminary conclusion: At least for the first year of operation the thickness of LH2 absorber will not need to be changed. It will be required to change the absorber to other materials to investigate their qualities MICE aims at observing cooling precisely and measure the equilibrium emittance precisely (relevant for cooling ring and Muon collider) Changing the thickness of absorber will come when investigating operation with higher gradients and will come along with other major hardware modifications (to the RF)