Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Introduction and performance evaluation of a new data acquisition
system for germanium detectors
H. Takeda,∗1 H. Baba,∗1 N. Fukuda,∗1 H. Suzuki,∗1 Y. Shimizu,∗1 M. Yoshimoto,∗1 Y. Togano,∗1 andK. Yoshida∗1Isomer tagging is an essential technique for unambiguous particle identiﬁcation (PID) in BigRIPS1) andis achieved by detecting characteristic γ rays emitted
from known short-lived nuclear isomers. Currently, twoclover-type high-purity germanium (Ge) detectors areused for this purpose. Data acquisition (DAQ) for
Ge detectors has traditionally been performed with aCAMAC-based system. However, many CAMAC modules are aging and becoming diﬃcult to maintain due
to discontinued production. Further failure of the critical modules could interfere with the isomer tagging.
Therefore, considering an alternative DAQ system has
been an urgent issue recently.
Mesytec MDPP-162) is a promising candidate for thenew DAQ system. It is a VME-based, 16-channel digital pulse processor with a fast high-resolution timeand amplitude digitizer. Input signals are ﬁrst ampliﬁed, ﬁltered, and then digitally reconstructed ina hardware component, followed by detailed analysis
in a digital signal processing unit implemented in aﬁeld-programmable gate array (FPGA). Among several FPGA software modules available for the MDPP16, the standard “SCP” module suits our purpose. It
delivers timing and amplitude information for standard charge-integrating preampliﬁer signals with 16-bit(64k) resolution. It can replace all the conventional
shaping ampliﬁers, peak-sensitive ADCs, timing ﬁlter
ampliﬁers, constant fraction discriminators, and TDCs.
First, we examined the ADC and TDC performanceof the MDPP-16 SCP. The integral non-linearity (INL)of ADC was less than 0.1%, while the diﬀerential nonlinearity (DNL) of ADC was less than 4% over the 20%–90% range of full scale. The INL and DNL of TDC wereless than 0.001% and 0.03%, respectively. Time slewing
with respect to the pulse height was less than 0.4 ns.
These performance values were comparable to or better
than the current system.
Figure 1 shows a γ-ray spectrum of a 60 Co source
measured with the MDPP-16 SCP by the Ge detector
used in the actual isomer tagging.
FWHM resolution of the 1332-keV peak was typically
4.7 keV. This value was mainly due to the accumulationof radiation damage to the Ge crystals over the years
and was the same as that measured with the currentsystem.
The isomeric γ-ray measurement using the MDPP16 SCP was performed during the BigRIPS tuning thisautumn. Figure 2 shows an atomic number (Z) versus mass-to-charge ratio (A/Q) PID plot for a 52 Cacentered radioactive isotope (RI) beam.
∗1

RIKEN Nishina Center- 78 -Fig. 1. γ-ray spectrum of a 60 Co source measured with theMDPP-16 SCP.

Fig. 2. Z versus A/Q PID plot of the 52 Ca-centered RI
beam. Delayed γ-ray energy spectrum of the 54 Sc isomer
is overlaid in the upper right corner.

The delayed γ-ray energy spectrum of the isomeric
nucleus 54 Sc is overlaid in the upper right corner. The
presence of a distinct 110-keV peak indicates that theMDPP-16 SCP can be used for isomer tagging.
Finally, we examined long-term stability, which isalso important for practical operations. The energy calibration was stable within 0.5 keV over several days if
no setting was changed. However, when we changed thesettings, such as thresholds, the calibration was occasionally shifted from a few keV to a maximum of 10 keV.
Based on oscilloscope observations, it is suspected thatthe baseline may shift when the settings are changed.
A detailed investigation is in progress. While this issue
must be resolved, the MDPP-16 SCP has the suﬃcientperformance required for the new DAQ system of theGe detectors. We plan to replace the current CAMACbased system with the new system as soon as possible.
References
1) N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B
317, 323 (2013).
2) https://www.mesytec.com/products/nuclear-physics/
MDPP-16.html .