Los espacios de acción y reflexión Propósito:

(GREAT) spectrometer

Situated at the focal plane of ritu - see Figure 3.0.1 - the Gamma Recoil Electron

Alpha Tagging (great) spectrometer [27] is a composite detector array consisting

of two Double-sided Silicon Strip Detectors (dssds), a MultiWire Proportional Gas-

FIGURE 3.3.2: Top-down view of the ritu gas-filled separator showing the QνDQhQν ion optical magnetic configuration and beam stop region (indicated by

the red line).

of 28 Sipin diode detectors. The arrangement of these detector systems is shown in

Figure 3.4.1. To detect high energyγ rays, a high-efficiency segmented clover-type HPGe detector is situated directly above thegreat spectrometer.

3.4.1

Multi-wire proportional counter (MWPC)

The multi-wire proportional counter (mwpc) is a type of gaseous ionization detector

used to count particles of ionizing radiation. As it can measure the energy of incident radiation, _mwpcs are widely used where it is necessary to distinguish between dif- ferent radiation types, for example α and β particles. Practically, a mwpc consists

of a collection of thin, parallel and equally spaced wires, sandwiched symmetrically between two cathode planes.

FIGURE 3.4.1: Schematic drawing of thegreatspectrometer showing the arrange-

ment of the silicon and germanium detectors. The _dssds are then highlighted in red. Recoiling nuclei would pass through themwpc (not shown) and the enter the

detector system from the left. Original image courtesy of Ref. [27].

The anode wires are grounded and as a negative potential is applied to the cath- ode an electric field is established. This field is then approximately uniform between the electrodes, except for the region near the wires where it becomes distorted. As a charged particle passes through the gas volume, charges are liberated in ionisation events and the particle will leave behind it a trail of electron-ion pairs. Electrons will then drift to the nearest wire causing a voltage pulse to be recorded.

3.4.2

Double-sided silicon strip detectors (DSSDs)

The_dssds are used to measure the energy, time and position of implanted ions and of their subsequent radioactive decay products (protons, alpha and beta particles). A dssd consists of two 60 ×40 mm2 active windows with 200 individual strips

at a pitch of 1 mm in both directions, giving a total of 4800 independent pixels. Approximately 80% of the distribution of fusion products in the focal plane is then covered by the active area of the dssd. The array of Si pin diode detectors then

surrounds the perimeter of the two_dssdwindows in order to measure the energy of conversion electrons emitted during the decay of implanted ions.

3.4.3

Silicon PIN diode array

Situated between thedssds andmwpcis an array of 28pindiode silicon detectors,

as shown in Figure 3.4.1. A pin diode is a (photo)diode with a wide, lightly doped

near intrinsic region placed between the (typically heavily doped) p-type and n- type semiconductor regions. The utilisation of such a near intrinsic region makes a pin-type diode more suitable for photodetection applications. Each diode has a

thickness of 500 µm and an active area of 28×28 mm2. Although their primary function is the detection of conversion electrons andβ decays, the array can also be used to reconstruct energy information ofα particles which escape the dssds.

Anαparticle emitted from thedssds towards thepindiode array in the upbeam

direction will deposit its energy shared between the dssds and the array. Thus

the full energy of the α decay can be reconstructed, although the accuracy of this technique is limited by energy losses in the two detectors. This add-back technique is a powerful method of increasing statistics, as typically 40% of events will escape the _dssds.

3.4.4

Planar Germanium (HPGe) strip detector

The role of the planar double-sided germanium strip detector is to measure the energies of X-rays and low energy γ transitions; providing positional information that can be correlated with data from the other great detectors. The detector is

mounted approximately 10 mm downbeam of the dssds and is housed in its own

cryostat with a thin beryllium3 _{entrance window. To minimise attenuation of low-}

3_{Because of its low density and atomic mass, beryllium is relatively transparent to X-rays and}

energy photons the detector is mounted inside _greatvacuum chamber. Like that of the dssds, the detector has an active area of 60×120 mm2.

As high-energyγ rays will pass through the detector, to be absorbed by the sur- rounding clover detectors, the planar detector’s (relatively) low thickness of 15 mm gives a maximum efficiency at < 100 keV. To allow for spatial correlations to be made with the dssds, a strip pitch of 5 mm in both x and y directions gives a

total of 288 pixels. Like the four clover detectors, the planar detector utilises the

v 0.5 µs time-of-flight from _jurogam to implantation in the _dssds to look for isomer-delayed transitions.

3.4.5

Clover detectors

Clover-type silicon detectors are so called because each detector consists of a seg- mented assembly of four co-axial n-type HPGe crystals resembling the shape of a

four-leaf clover. Four clover detectors (totalling 16 crystals) are situated left, right, above and downbeam of the focal plane. Housed outside of the vacuum chamber, as shown in Figure 3.4.1, the clover-type detectors can be used for the detection of high-energy γ rays emitted from isomeric states.