RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

Pulse height defect in the sc CVD diamond detector versus theapplied electric ﬁeld measured with ﬁssion fragments from 252 CfO.Beliuskina,∗1 N.Imai,∗1 H.Haba,∗2 S.Heinz,∗3 M.Kis,∗3 and M.Traeger∗3
The response of semiconductor detectors to the energy lost by heavy ions is complicated because the kinetic energy of the ion is not completely converted into
an electric signal. The emerging pulse-height defect(PHD) depends on the energy, mass, and charge of theion as well as on the applied electric ﬁeld. The PHD
is deﬁned as the energy diﬀerence ΔE between the kinetic energy Ek of an ion incident in the detector andthe apparent energy EDD derived from the measured
electric signal.
Most detailed studies of PHD were performed for
Si detectors.1–3) Another promising detector material
that appears to extend the possible applications forheavily-ionizing particles compared to Si is chemical
vapor deposition (CVD) diamond.4,5) However, theuse of diamond detectors (DDs) as detectors for lowenergetic heavy ions is still at its beginning. There are
only few publications about PHD in DD.6–8)
The use of DD as ﬁssion fragment detectors wasstudied by Fregeau et al.7) The authors concluded that
all their DDs showed inadequate energy resolution andsuﬀered from extensive PHDs. The detected pulse
height never exceeded about 30 % of what they expected from a simulation of the stopping of 252 Cf ﬁssion fragments in DD.
In an earlier experiment at the FLNR, Dubna, weinvestigated the PHD for ﬁssion fragments of a 252 Cf
source at diﬀerent electric ﬁelds.8) Values between 0.7−2.5 V/μm were used. The charge collection slowly
increased towards saturation, but it was not yet saturated at 2.5 V/μm. At 0.7 V/μm, the PHD is about
70 % of the kinetic energy for heavy (HF) and light(LF) fragment groups of 252 Cf and about 59 % at 2.5V/μm.
The aim of our new measurements at the HotLab,
RIKEN, was to study the dependencies of the PHD
caused by ﬁssion products of 252 Cf versus the appliedelectric ﬁeld up to the highest possible bias and tocompare these results with those of previous studies
obtained with another single-crystal (sc) CVD DD.
The experiment was carried out at the HotLab,RIKEN. The dependence of the peak positions forthe ﬁssion fragments of the 252 Cf source of 0.5 MBq
activity on the electric ﬁeld were measured with a
sc DD manufactured at the GSI detector laboratory.
The charge sensitive preampliﬁer CSTA2, the ORTEC
spectroscopic ampliﬁer, and the multichannel analyzer
Kromek K102 were used. Detector dimensions were 3∗1∗2∗3CNS, University of Tokyo
RIKEN Nishina CenterGSI, Darmstadt- 210 -× 3 mm2 with a thickness of 200 μm. The DD had
CrAu electrodes with thicknesses of 40 nm. The 252 Cf
source was placed at a distance of 10 cm from the DD.
The energy calibration was performed with α particles
from a mixed source that has energies of 4.780, 5.480,
and 5.795 MeV. The calibration of the electronics wasperformed with a precision pulser. The mean energiesof the heavy and light fragment groups of 252 Cf weremeasured. The diﬀerence between the measured energies and the values reported in the literature1,9) was
deﬁned as the PHD.
The working electric ﬁeld for DDs is about 1.0−2.0 V/μm10) although its breakdown value for DDs
is much higher. All the charge created by α particles
with energy of about 5 MeV has already been collected
at 0.5 V/μm, and the pulse height remains the sameat all values of the electric ﬁeld from 0.5 to 5.0 V/μm(applied in our experiment). In contrary, for ﬁssion
fragments of the 252 Cf with an increasing electric ﬁeld
from 1.0 to 2.0 V/μm, the pulse height increased by8 %, indicating that the created charge is not fully collected. Further, PHD dependency on the applied electric ﬁeld, which has a quite steep slope in the region
1.0−2.0 V/μm goes almost to its saturation between
2.5 and 3.5 V/μm. Namely, the PHD decreases in thisregion only by about 1 % for LF and about 2 % for
HF. It was not possible to use an electric ﬁeld higher
than 3.5 V/μm for ﬁssion fragments of 252 Cf because
occasional breakdowns started to occur.
We conclude that the application of an electric ﬁeld
higher than 2 V/μm slightly improves the charge collection eﬃciency of DDs for ﬁssion fragments of 252 Cf.
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