Policy of the design

Box-shaped NaI(Tl) detectors were chosen to satisfy the follogin criteria:

• Good efficiency
• High granularity
• Flexibility

Since the unstable nuclei are delivered as fast secondary beams with relatively low beam intensities (typically 0.1 cps to 1 kcps), the γ-ray yields of the nuclei of interest tend to be rather low in in-beam γ-ray spectroscopy experiments. Thus, in order to perform the experiments as efficiently as possible, a γ-ray detector array with a high intrinsic efficiency is required.

One futher important requirement is the ability to achieve a high angular resolution since the energies of the γ rays emitted from fast-moving nuclei are shifted due to the Doppler effect, which is dependent upon the angle of γ-ray emission.

In order to utilize DALI2 effectively in different types of experiments, such as inelastic scatterings or recoil shadow methods, a simple arrangement has been adopted in which the detectors are mounted on a support frame; each detector is supported by a 3-mm-thick aluminum plate, and are spaced at 5-cm intervals. This allows flexibility for changing the detector configuration between experiments, if necessary.

NaI(Tl) Detectors

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There are three types of NaI(Tl) detectors used in the DALI2 array; one type manufactured by SAINT-GOBAIN, one by SCIONIX. The third, old DALI1 type crystals, is manufactured by BICRON and SAINT-GOBAIN. The crystal sizes are 45 x 80 x 160 mm³ (left photograph) and 40 x 80 x 160 mm³ (right photograph) for the SAINT-GOBAIN and SCIONIX types, respectively. The DALI1 type crystals have dimensions of 61 x 61 x 121.9 mm³. Presently 88 SAINT-GOBAIN, 50 SCIONIX, and 88 DALI1 type detectors are employed in the array, totalling 226 detectors in the standard configuration of DALI2⁺. Each scintillator is coupled to a 38-mm-Φ HAMAMATSU R580 or 50.8-mm-Φ HAMAMATSU R6231 photomultiplier tube (PMT) and encapsulated in a 1-mm-thick aluminum shell. A typical intrinsic resolution is about 9% (FWHM) at 662 keV (¹³⁷Cs stationary source).

Old DALI2 Detector arrangement

DALI2 is composed of sixteen layers of detectors in total, all of which are orientated perpendicularly to the beam axis. The detectors cover the angular range between 20 and 160 degrees. Each layer consists of 6-14 detectors, which are supported on 3-mm-thick aluminum plates mounted on a support frame and spaced at 5-cm intervals. The distance from the target to the center of each detector is about 30 cm for the detectors located at a polar angle of 90 degrees relative to the beam line (i.e. in the layer closest to the target position). The standard DALI2 configuration gives a good angular resolution for the γ rays; approximately 7 degrees at γ-ray emission angles of 70 degree, where the Doppler-shift effect is largest in in-beam experiments with β = v/c = 0.3. The DALI2 configuration with 186 detectors contains 66 SCIONIX, 88 SAINT-GOBAIN, and 32 DALI1-type detectors.

New DALI2⁺ Detector arrangement

DALI2⁺ is composed of ten layers of detectors in total, of which 162 are orientated perpendicularly to the beam axis and 64 in a wall configuration parallel to the beam axis. The detectors cover the angular range between 20 and 160 degrees. Each layer consists of 10-32 detectors, which are supported on 3-mm-thick aluminum plates mounted on a support frame and spaced at 5-7cm intervals. The distance from the target to the center of each detector is about 30 cm for the detectors located at a polar angle of 90 degrees relative to the beam line (i.e. in the layer closest to the target position). The standard DALI2sup>+ configuration gives a good angular resolution for the γ rays; approximately 7 degrees at γ-ray emission angles of 70 degree.

Electronics

The typical electronics setup for DALI2 is shown in the figure on the right. A multichannel power supply system (CAEN SY1527) provides around 1200 V to each PMT. The signal from each PMT is fed into a shaping amplifier (CAEN N568B) and split into two separate signals for energy and timing measurements. The signals for the energy measurements, OUT of N568B, are passed to a peak-sensing analog to digital converter (CAEN V785 ADC). A fast-out signal (FOUT from N568B) is fed into a constant-fraction discriminator (CAEN V812 CFD) and an output signal is fed into a time to digital converter (CAEN V1190A TDC) as a stop signal.

For data acquisition, a trigger signal (γ trigger) is generated by the OR logic of a CFD, which is used as a gate signal for an ADC and a start signal for a TDC. Data acquisition is performed by the RIBF DAQ system (Babirl) and on/off-line analysis is done using ANAROOT.

In in-beam experiments, the trigger signal is obtained from a coincidence signal that is generated by combining the γ-ray and beam triggers.

Full-energy-photopeak efficiency and energy resolution

The figure on the right shows the full-energy-photopeak efficiency of the DALI2 array as a function of γ-ray energy in the laboratory frame (β = 0.0). The efficiency curve was obtained by Monte Carlo simulations with no target chamber. More realistic values, with the target holder (cell) and the target chamber in place, may be about 20% lower than calculated values. In fact, the full-energy-photopeak efficiency was measured to be about 24% (FWHM) at 662 keV (¹³⁷Cs), with the target chamber and the target cell (CRYPTA).

The energy resolution was measured to be about 9% (FWHM) at 662 keV; this is same as the averaged value obtained for all detectors.

Full-energy-photopeak efficiency and energy resolution in in-beam experiments

The figures below show full-energy-photopeak efficiencies for different β values as a function of γ-ray energy, obtained by Monte Carlo simulations with no target chamber. The efficiencies are those obtained after applying Doppler-shift corrections. The figure on the left is the result of simulations for DALI2 (160 detectors) with β = 0.3, while the one on the right is for DALI2+ (180 detectors) with β = 0.6.

The simulated energy resolution at 1 MeV is about 8% and 13% for β = 0.3 and β = 0.6, respectively, after applying Doppler-shift corrections. These values may change in reality as they depend on the intrinsic and angular resolutions of the detectors, and the energy differences between the beam paricles when the γ rays are emitted.

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