The LISE program:Version 4.0.17 (20.01.00)

You may get the new version of the code using the reference
http://http:"// - after via HTTP(new possibility - ZIP-file) or FTP (as it was before "Self-extractor exe-file")

Different profiles of a degrader (wedge) in the intermediate dispersive focal plane

The code used in previous versions only achromatic profile of wedge. Nowadays, There are 4 possible profile of a degrader (wedge) in the intermediate dispersive focal plane in the new version:

· achromatic
· monochromatic
· homogeneous
· user-defined

Figure 1.

All profiles in the code suggest wedge-shape where only an angle of wedge is changed. User may find a negative angle of wedge in the case of negative momentum dispersion of the first path of spectrometer. It means a wedge is turned on 180 degrees.

In the example [1], a fragment beam of 19Ne with 600 A MeV with a velocity spread of ~1.5% penetrates through differently shaped degraders all having the same thickness at the optical axis (d/r=0.5). In the left panel of Figure 2, the profiles of the degraders are chosen such that the thickness is constant along the x coordinate (homogenous degrader). In the middle panel, the slope of thickness preserves the achromatism of the ion-optical system (achromatic degrader), and in the right panel, the profile matches the velocity dispersion in the x direction and thus bunches the energy spread as well as range distribution (monoenergetic degrader). The position distributions at the final focus in Figure 2 suggest that the achromatic degrader is superior for spatial isotopic separation because it is shaped so that the image size is independent of the incident momentum spread of the fragments. Interesting applications are suggested by the use of a monoenergetic degrader, especially if a narrow implantation distribution proves to be of importance, e.g. for implantation in thin detectors used in nuclear decay spectroscopy or in biomedical treatments.

Figure 2. Phase-space imaging of differently shaped degraders within the achromatic ion-optical system. The results for a homogeneous, an achromatic, and a monoenergetic degrader are given. All degraders have the same thickness on the optical axis (d/r=0.5) [1].

Optic and degrader When introducing an energy degrader in the intermediate dispersive focal plane of dipole D1 general conditions for double achromatism of the spectrometer change into the following [2]: ,


where F is equal to the ratio (?p/p)B /(?p/p)A of the momentum dispersions at the exit the entrance of the degrader. F depends on nature, thickness and shape of the degrader. Two cases can be practically considered F?1 and F=1.

For F?1, the two above matching conditions are thus only obtained by optics tuning, especially varying the dispersion coefficient (X/?)B. It expects to use such a degrader with F<1 in order to reduce momentum spread (or energy spread) in the second part of the spectrometer and particularly at the final focus point, to minimize range dispersion of selected products stopped in solid state detectors.

In the second case (F=1), optic matching conditions remain the same as without using of degrader . The field of all the magnetic elements of the second section has only to be scaled to the proper value B2/B1, calculated for a chosen value A3/Z2 of the selected nuclei. One however has to build a special shaped degrader (achromatic degrader) in order to maintain the condition F=1 (i.e. the same relative momentum spread before and after the degrader).

The next problems with degrader [2] was considered in the code:

· Image broadening in a thick degrader.
· Energy straggling in the degrader
· Focal point magnification
· Intrinsic aberration
· Degrader defect (1% default)
  Examples of calculation with degrader of different profiles
For example the default configuration after loading of the code was used only the Al-wedge was installed with the thickness 200 micron. The magnetic rigidity of dipoles was calculated for 36Ar.

Calculations of transmission was made only for all Ar-isotopes:
Profile of Wedge
Number of Ar-isotopes
Almost events
36Ar events


Wedge selections plots for different cases of degrader profile


Range distributions in the silicon detector for different cases of degrader profile


[1] H.Geissel, G.Münzenberg, K.Riisager, "Secondary exotic nuclear beams", Annu. Rev. Nucl. Part. Sci. 45 (1995) 163-203.

[2] R.Anne, D.Bazin, A.C.Mueller, J.C.Jacmart and M.Langevin, "The achromatic spectrometer LISE at GANIL", NIM A257 (1987) 215-232.