The total cross section of the20Ne(p,γ)21Na reaction was measured at Elab
p = 330 keV. A list of possible γ-rays from transitions in this reaction is given in Table10.1. According to estimates from Rolfset al.(1975) [2], direct capture into the 2425-keV state should dominate the cross section at these energies. ThisJπ= 1/2+ state decays purely to ground via the isotropic emission ofγ-rays of M1 multipolary and, hence, DC primary and secondary signals at 313 and 2425 keV, respectively, were expected. However, subthreshold resonant capture into the 2425-keV state should also contribute∼14% to the total yield of the 2425-keVγ-ray peak at this beam energy. There is no HPGe singles spectral analysis method to separate the capture γ-rays resulting from the subthreshold resonance and those coming from the 2425→0 keV secondary transition following direct capture into the 2425-keV state in 21Na. Instead, a careful comparison of the detected counts in the 313- and 2425-keV peaks should reveal (following angular correlation, coincidence summing and full-energy peak efficiency corrections) an over-abundance of counts in the 2425-keV peak, caused by subthreshold resonant capture. Therefore, detection of the DC primaries would be necessary to determine the relative contributions of DC and subthreshold resonant processes to the total cross section.
The above experimental constraints required the best possible arrangement for the detection of DC primaries and accurate measurement of the effective energy of the beam within the target at all times. Therefore, a dual, HPGe detector arrangement was selected to facilitate this measurement. Detectors were placed at 55◦ and 90◦ with respect to the target in a close geometry. By placing a detector at 55◦, angular correlations between the beam and DC primary emission directions [160] were minimized. The detector at 90◦minimized Doppler-shift effects on the measuredγ-ray energies in all spectra. This arrangement allowed for an accurate estimation of the effective energy of the beam via the centroid location of DC primaries in
the 90◦ HPGe singles spectra. The effective energy is (by definition) the energy of the protons within the target at which half of the yield for the full target thickness is obtained. The detectedγ-ray energy in this case is shifted to lower values because of energy loss by the proton beam within the target before the direct capture process occurs. If the decrease in cross section over the target thickness is linear, the effective energy (following this energy loss) is given by [72],
Eeff =E0−∆E+∆E " −σ1σ2 −σ2 + σ2 1+σ22 2(σ1−σ2) 1/2# (10.1)
where E0 is the initial beam energy,∆E is the target thickness, and the cross section termsσ1 and σ2 are evaluated at the energiesE0andE0−∆E, respectively. Alternatively, if the cross section drops exponentially with energy, the effective energy can be found by first taking the form of the cross section asσ(E) =AebE where A and b are constant coefficients found via fits to the cross section curve. According to the effective energy definition given above, we can make the following comparison of yields over the target thickness,
1 Z E0 E0−∆E σ(E)dE= 2 Z E0 Eeff σ(E)dE. (10.2)
Inserting the functional form forσ(E) and integrating we find,
ebEeff= e bE0
2 1 +e −b∆E
. (10.3)
After simplifying, we find an expression forEeff,
Eeff=1 b ln(1 +e−b∆E) +bE0 −ln(2) . (10.4)
Cross section estimates using the S-factor data in Rolfs et al. (1975) [2] and the expressions for effective energy in Eqns.10.4and10.1gave identical results−therefore linear estimates were employed throughout. Hence, by carefully measuring the energies of the DC primaryγ-rays, any shift toward lower values implied a gradual decrease in the effective energy of the beam within the target. Physically, this meant that the implanted20Ne was steadily driven deeper into the backing following long-term beam deposition.
Proton beams from the JN accelerator of ∼ 130 µA were incident on three 20Ne-implanted tantalum backings: Ne-Ta targets #39 (11.0 C, #37 (7.8 C), and #15 (11.2 C). This accelerator and its terminal voltage stabilization system are described in Sections 2.2 and 9.2. Therefore, the energy resolution of the beam was∼2 keV. The data set for each of these backings took∼20 hours to collect. Initially, titanium backings were employed, but high beam-induced background signals from the Elab
a) c)
b)
Figure 10.1: Photographs of the primary components of the 55◦-90◦ target arrangement. a) The angled e−-suppressor tube, insulating ceramic section, and beam-collimation pipe are given. b) Angled beam pipe end with its elliptical flange is shown with the lead shield pulled back (at left). c) An overview showing the placement of each HPGe detector with respect to the target inside the lead shield (top-half is removed). the 14N(p,γ)15O reaction increased the Compton continuum in the low-energy portion of the spectrum to unacceptable levels and disqualified their further use. The target station and detector arrangement described below were used.
Section 10.3: 55◦-90◦ Detection System
10.3.1: Target Station
A completely new target station was designed, machined, and installed to accommodate the dual de- tector arrangement introduced in Section10.2. This target geometry needed to fulfill several practical and experimental requirements. It needed to accept, seamlessly, the water-cooled LENA target holder and its 5.21-cm-diameter sealing-shoulder (see Section 2.4.3 and Ref. [79]). The target holder and experimental backing both needed to be positioned as closely as possible to the HPGe detector to avoid significant loss of full-energy peak efficiency. Features for adequate alignment with the beam axis, beam collimation, and target current e−-suppression were also necessities. Finally, the new design needed to be compact enough to fit within a thick-walled lead detector shield for passive reduction of environmental background. The target
station given in Fig.10.1satisfies all of the above requirements.
A new section of beam pipe was cut at a 55◦ angle and an elliptical flange-fitting was welded to this angled beam pipe face (see Fig.10.1b). This allowed for easy accommodation of the standard LENA target holder. Also an angled, copper e−-suppression tube was machined to ensure proper current integration on the target (see Fig.10.1a). This suppressor had an inner diameter (ID) of 12.7 mm and its knife-edge sat at 1.6 mm from the face of the target backing. Throughout this experiment, this suppressor tube was biased to -300 V.
Adequate beam collimation was also crucial during all data collection to ensure that only the implanted region of our targets received proton beam. Also, the suppressor tube described above could not be within the line of sight of the beam or else negative charge could reach the target backing and sputtering effects could arise. To avoid any of these unwanted effects, a 1.9 cm ID molybdenum collimator was placed∼ 5.08 cm upstream from the suppressor tube and target face. The collimator, suppressor tube, and target holder were all electrically isolated from each other and periodic continuity checks were conducted to ensure proper current integration at all times. This system was self-aligning and resulted in an elliptical beam profile on target whose major axis was 0.95 cm.
Because the target was tilted by 35◦ with respect to the standard, 0◦ configuration, a subtle but conse- quential correction to the measured target thickness must be included in further analysis. This is caused by the geometry of the target orientation with respect to the beam (see Fig.10.1) and the resulting (sin (35◦))−1 correction factor was included in all target thickness calculations.
A lead-walled, dual detector shield and accompanying support stand was designed, molded, machined, and installed in-house for this experiment. The walls of this shield azimuthally surrounded the crystals of both HPGe detectors with at least 9.53 cm of lead (see Fig. 10.1c). The detectors, lead shield, and accom- panying mounting hardware were all secured to a rail-mounted aluminum platform on roller bearings. This allowed for easy access to the target, and precise and reproducible placement of the detectors with respect to the target center throughout data collection. During final assembly of the target station, both detectors were aligned to within 0.5 mm of the target center. It should be noted that during data collection, insulating tape was applied to the rear of the target holder to prohibit electrical contact between it and the aluminum crystal can of the 55◦ detector. This introduced an offset between the detector and target axes of
∼1 mm
10.3.2: 55◦-90◦ DAQ Electronics and Software
55◦-90◦ Electronics
As in prior experiments at LENA, detected signals were shaped and processed by a series of NIM and VME bus modules, output to a single board computer, and then sorted in theJAMDAQ software suite. In essence, singles spectra were collected from each HPGe detector while a single plastic scintillator panel (see Section9.3and Ref. [144]), placed on top of the lead shield, provided a cosmic ray veto condition. No other coincidence detection provisions among the HPGe detectors were made since their vanishingly small coinci- dence efficiency was assumed to not be experimentally viable. Also, systems for beam current integration remained unchanged from the description in Section9.3.
A schematic diagram of the necessary DAQ electronics for this detection system is given in Fig.10.2. The crystals of both HPGe detectors were biased to their necessary depletion voltages using a dual high-voltage power supply. Each detector output two identical signals from their preamplifiers: one signal served as a timing pulse, the other was used as an energy pulse.
This energy pulse was split via a BNC tee connector; each of these energy signals in turn served as input to high (High) and low (Low) gain spectroscopic amplifiers for further shaping. The Low gain amplifier allowed for hit detection and storage up to∼9 MeV. This capability gave spectroscopic information on high- energyγ-rays from beam-induced contaminant reactions. The High gain amplifier provided the ability to expand the low-energy (.3 MeV) portion of the spectrum, where peaks of interest from the20Ne(p,γ)21Na reaction reside. All four of these shaped unipolar energy signals (Highand Lowfrom each detector) were then sent to the ADC for processing by the SBC and storage inJAMhistograms.
The timing signals from each detector were sent into separate channels of a quad TFA for shaping; these shaped signals were then discriminated via a quad CFD. The CFD thresholds for both detectors were set at
∼230 keV. These discriminated pulses were input into separate gate-and-delay generators (GDG) as trigger signals for the creation of individual HPGe timing gates from each detector. Each of these HPGe gates were sent to separate fan-in/out modules to generate two logic pulses. One pulse was sent to a channel on the multichannel scaler module, which counted gates within each detector, while the other was sent to a master fan-in/out module. Upon receipt of an input pulse from either detector, this module generated two output signals: one that was then delayed by ∼11 µs by a subsequent GDG module and used as an OR timing signal for the master gate input on the ADC; the other was used for cosmic-ray veto purposes.
Hits within the plastic scintillator panel placed above both HPGe detectors were processed using the same modules and methods described in Section 9.3. Coincident detection between γ-rays in either HPGe detector and hits in the scintillator panel was facilitated via the master timing signal discussed above. This
55 HPGe 90 HPGe
55 Spec. Amp. HI/LO Gain Ortec 572 90 Spec. Amp HI/LO
Gain Ortec 572 Ortec 863 Quad TFA Ortec 935 Quad CFD ADC Muon panel 6 us GDG Ortec 416A Caen N568B Spec. Amp. SIS Scaler 90 GDG Phillips 794 Ortec 567 TAC 55 GDG Phillips 794 Pulser Ortec 448 Dual HV Bias Supply
Ortec 660 55 Ge Gates (Ch. 3) 90 Ge Gates (Ch. 2) Pulser Gates (Ch. 10) 90 Ge Gate Output 55 Ge Gate Output Pulser Gate Output Trigger Trigger Trigger Trigger Trigger Trigger Delay Delay Delay Gate HV Bias Supply Ch. A (+2400 V) In. 2 In. 1 In. 2 In. 1 Canberra 2058 63 ns Delay Ortec 425A 63 ns Delay 55 Fan In/Out LeCroy 429A 90 Fan In/Out LeCroy 428F Delayed ADC Signal GDG Phillips 794 Delayed ADC Gate Output Trigger Trigger Delay HI/LO Ch. 13/14 HI/LO Ch. 5/15 Ch. 7 Ch. 1 Ch. B (+4000 V) Start Stop Caen V895 LED OR XOUT FOUT Master Fan In/Out LeCroy 428F Beam current Current Integrator Ortec 439 Beam Pulsing Circuit BCI (Ch. 5) Ch. 6 Preamp Preamp Uni. Out Uni. Out Energy Out Energy Out Time Out Time Out Test Test
Figure 10.2: A block diagram of the 55◦-90◦ detection system electronics setup. Black dots denote wire nodes; otherwise, wire crossings are not physical. See text for further signal processing details.
signal acted as aSTARTpulse for the scintillator TAC module, while the timing signal from the scintillator provided aSTOPcondition. The TAC coincidence signal was processed by the ADC, SBC, and sorted into aJAM histogram where a veto gate could be drawn.
The system employed to monitor DAQ dead time was modified slightly. A single pulser output a 1 Hz signal that was split via a BNC tee. Each of these signals were then sent to the test inputs on each of the detector preamplifiers and displayed as artificial peaks in their respective HPGe singles histograms inJAM. Simultaneously, the pulser trigger output signal was sent to a GDG module and a pulser gate signal was
generated for scaler input. Therefore, dead time was determined via a comparison of counts in each of the artificial pulser peaks with the total number of pulser gates counted in the multichannel scaler.
55◦-90◦ JAM DAQ Software
A new JAM software script was written that incorporated all of the necessary variables and logic to accompany the detector electronics setup described. The JAVA sort routine (Lena5590sort.java) used to analyze all incoming signals and create histograms is given in full in Appendix SectionE.4.