MAMI

At the MAMI-facility, a tagged photon beam is provided in combination of the MAMI electron accelerator [53] and the Glasgow Tagged Photon Spectrometer [54].

Both will be discussed in the following.

Accelerator

The basis of the MAMI accelerator are an injector LINAC and a cascade of three RaceTrack Microtrons (RTMs), which are providing an electron beam of 100µA up to an energy of855 MeV. The facility was upgraded in 2006 by adding a fourth stage, theHarmonicDouble SidedMicrtron (HDSM), providing a beam energy up to1.5 GeV.

At first, an electron source, a thermionic DC-gun, supplies100 keVelectrons to the LINAC. The LINAC again supplies electrons with an energy of 3.97 MeV to the RTMs. The electrons are accelerated stepwise by each RTM with energies according to Tab. 3.3. The three RTMs have a similar functionality and the same basic setup

Parameter Unit Injector RTM RTM RTM

LINAC 1 2 3 HDSM

injection

[MeV] - 3.97 14.86 180 855

energyE_inj extraction

[MeV] 3.97 14.86 180 855 1500 energyE_ext

magnetic field [T] - 0.103 0.555 1.284 1.53−0.95

number of turns - - 18 51 19 43

energy gain∆E

[MeV] - 0.60 3.24 7.50 16.58−13.66 per turn

LINAC length [m] 4.93 0.80 3.55 8.87 8.6and10.1

RF-system

[GHz] 2.4495 2.4495 2.4495 2.4495 4.8890|2.4495 frequency

beam energy

[keV] 1.2 1.2 2.8 13.0 110.0 spread (1σ)

Table 3.3: General MAMI parameters [53].

which is depicted in Fig. 3.10. In principle, an electron beam injected into a RTM is recirculated several times through the RTM while the energy is increased in each cycle. The electron beam passes theRadioFrequency (RF)-cavities with a frequency of about2.45 GHz. This results in an energy gain of ∆E for the electrons which is

Figure 3.10: Basic setup of a RTM [53].

in the order of a few MeV. Two dipole magnets provide each a constant magnetic field. Both magnets have the purpose to deflect the electron beam by approximately 180^◦ back to the RTM. The bending radius R of an electron beam with a velocity β =v/c≈1increases linear with the beam energy because of the constant magnetic field and amounts to

R = β·E

e·c·B . (3.1)

Furthermore, it has to be ensured that the electrons stay within the same phase of the alternating voltage in the linear section. Hence, the difference in time between each cycle has to match an integer multiple of the period of the accelerator frequency.

In addition, the RTM features an automatic phase correction which enables a rather small spread in energy of the electron beam. If a beam particle has a larger or lower energy than the energy of their corresponding cycle, the path in the bending sec-tions will be larger or smaller, respectively. Thus, the beam particle arrives respec-tively later or earlier in the linear section. Due to the fact that the electrons enter the high frequency with a different phase, the beam particle is under-accelerated or

over-accelerated, respectively, in the next cycle. This results in an energy correction.

Finally, after a fixed amount of cyclesN, the beam is extracted by a so-called kicker magnet with an energy of

E_ext =E_inj+N ·∆E. (3.2)

The MAMI-facility was upgraded in 2006 with the HDSM which is now the last section of the MAMI accelerator structure. The working principle is similar to the RTMs. The electron beam is accelerated stepwise to the final energy of up to1.5 GeV by two LINACs operated at2.45 GHz and 4.9 GHz. The beam is bended by four dipole magnets each time by ∼ 90^◦. It takes 43 turns within the HDSM before the beam is being extracted to the experimental areas. The scheme of the HDSM including LINAC-, bending- and focussing sections is depicted in Fig. 3.11. One has to note that the HDSM was not used for the experiment presented in this thesis.

Figure 3.11: Scheme of the HDSM of MAMI C [53]. The beam is accelerated to the maximum facility energy of 1.5 GeV by two LINACs and bended by four dipole magnets.

The Glasgow Photon Tagging Spectrometer

The beamtime test with the PROTO120 has the goal to investigate the response of the prototype to photons in the low energy regime. At the MAMI facility, the electron beam is converted into a continuous and collimated photon beam by a tagging system.

Electrons with a known initial energy E₀ impinge a radiator made of either copper or diamond. Hence, the photons are produced within the Coulomb field of the nuclei of the radiator via bremsstrahlung. The remaining momentum of the electrons is measured with the Glasgow Photon Tagging Spectrometer whose basic scheme is depicted in Fig. 3.12. The electrons are bended towards a focal plane by the magnetic

Figure 3.12: Basic Scheme of the Glasgow Photon Tagging Spectrometer [55].

field of a dipole magnet with a flux of B = 1.8 T due to the Lorentz force. The focal plane consists of 353 overlapping plastic detectors, each24 mmwide and2 mm

thick, read out via PMTs. The point of impact in the focal plane gives the remaining momentum of the electron which amounts to

p_e =R·q·B , (3.3)

with the known curvatureR. Then, the photon energy E_γcan be calculated with the initial beam energyE₀ and the energy of the deflected electronE_eby

E_γ =E₀−E_e. (3.4)

Almost monoenergetic photons can be selected out of the bremsstrahlung spectrum via a coincidence between the individual channels of the tagging system, which are digitized by a TDC, and a detector, the PROTO120 in case of the presented beamtime.

The width of the tagged photons is given by the energy resolution of the tagger and amounts to be below3 MeV depending on the energy. A set of collimators is placed 2.5 m downstream of the radiator (compare to Fig. 3.13). In case of the presented beamtime a collimator with a diameter of1.5 mmensures a small opening angle of 0.0172^◦ of the photon beam. Geometrical considerations lead to an upper limit for the diameter of the impinging beam on the PROTO120 of9.3 mm.

Figure 3.13: Schematic setup of the PROTO120 at MAMI.