Production and separation of positron emitters for hadron therapy at FRS-Cave M

Abstract

The FRagment Separator FRS at GSI is a versatile spectrometer and separator for experiments with relativistic in-flight separated short-lived exotic beams. One branch of the FRS is connected to the target hall where the bio-medical cave (Cave M) is located. Recently a joint activity between the experimental groups of the FRS and the biophysics at the GSI and Department of physics at LMU was started to perform biomedical experiments relevant for hadron therapy with positron emitting carbon and oxygen beams. This paper presents the new ion-optical mode and commissioning results of the FRS-Cave M branch where positron emitting ¹⁵O-ions were provided to the medical cave for the first time. An overall conversion efficiency of 2.9±0.2×10^{-4 15}O fragments per primary ¹⁶O ion accelerated in the synchrotron SIS18 was reached.

Keywords: Hadron therapy; In-flight separator; Ion optics; Positron emitter.

Fig. 1. Ion-optical calculation for FRS-Cave M.

The upper and lower half shows the beam envelopes and the magnetic elements of the FRS in the x- and y-plane, respectively. The beam envelopes (solid lines) are shown for a elliptical phase space with ε_x = 30 π mm mrad, ε_y = 20 π mm mrad, x₀ = 1.5 mm, y₀ = 2.2 mm and δp₀ = 0%. The dispersion line (dotted, black) represents a momentum deviation of 1%. For this case, the vertical scale represents the positive and negative coordinate in the horizontal x-direction. The vertical dashed line in Cave M indicates the position 161.38 m where the waist in x and y is fitted. The names of the different focal planes are indicated on top and follows the convention used at the FRS. The missing planes F4 and F6 belong to other branches of the FRS. The size of magnets correspond to their apertures (vertical scale) and lengths (horizontal scale). The elements between F8 and Cave M belong to the high-energy beam transport line of GSI, which was designed for primary beams; therefore they have smaller apertures than the FRS magnetic elements.

Fig. 2

(a) Particle identification scatter plot of ions arriving at Cave M measured event-by-event. Only a small fraction (of about 0.1%) of A/q = 2 is seen in addition to the fragment of interest, ¹⁵O. The main goal was to obtain a maximum intensity of ¹⁵O and therefore no additional separation with slits were applied to reduce the level of contaminants further. Simulations show that the most prominent contaminant is ¹⁴N produced in the production target located at F0. (b) The standard deviation of the momentum spread of the ¹⁵O is measured to be 0.6%. (c) The measured beam spot size on the detector located at the position indicated with a dashed line in Fig. 1 in Cave M has a standard deviation of 7.9 mm.