γ-ray beam characterization

A laser Compton scatter γ-ray beam was generated in a interaction chamber by employing a CO₂ laser (wavelength: 10.64 μm) operating at a low frequency of 1 kHz, with a pulse width of 50 μm (equivalent to 5 W laser power). This laser beam was collided with a 3.5 GeV electron in the SSRF storage ring, resulting in the production of quasi-monochromatic γ-rays beam with energies varying from 0.66 MeV to 21.7 MeV. The γ-ray beam flux ranged from 4.8×10⁵ ph/s to 1.5×10⁷ ph/s [1]. The main parameters of the SLEGS are listed in Table 1. The LCSS γ-ray beam was then directed through a vacuum pipeline, passing sequentially through a coarse collimator, fine collimator, and attenuator [20-23] before ultimately reaching the experimental hall. This well-controlled transport setup ensured precise delivery of the γ-ray beam to the experimental hall.

Figure 1 shows a diagram illustrating the online activation and offline measurements. The activation platform, featuring a multi-slot target holder, was strategically positioned behind the beam pipe exit. A silicon pixel imaging detector (MiniPIX) was used to facilitate beam spot imaging and reaction target localization. Additionally, a Φ 76.2 mm × 101.6 mm LaBr₃(Ce) detector was placed at the termination point of a LCS γ-ray beamline to measure both the γ-ray beam flux and energy. Figure 2 shows the detector response to γ-ray beam at collision angles of 124° and 132° using a 3 mm coarse collimator under 200 mm copper attenuation. The energy and efficiency calibrations of the LaBr₃(Ce) detector were performed using monoenergetic gamma rays from nuclear reactions (6.13 MeV, 9.17 MeV, 10.76 MeV, 17.6 MeV) and the radioactive source ⁶⁰Co [24-27]. The profile of a quasi-monoenergetic γ-ray beam overlaid on a continuous bremsstrahlung background was clearly visible. Using the unfolding method, the corresponding γ-ray energy spectrum without detector response was successfully solved, as shown in Fig. 2 (pink and blue). The details of the unfolding method for the γ-ray beam are presented in [28].

Fig. 1Full-screen View

(Color online) Schematic layout of the SLEGS beamline, online activation, and offline low background HPGe setup

Fig. 2Full-screen View

(Color online) γ-ray energy spectrum measured with a Φ 76.2 mm × 101.6 mm LaBr₃(Ce) detector. The spectrum was solved by the unfolding method. γ-ray energy spectrum of several laser-electron collision angles measured by a LaBr₃(Ce) detector for 124° (black) and 132° (red) (Unfolding γ-ray energy spectrum at the same angles)

Low background HPGe detector systems

The characteristic γ-rays emitted from a nuclide sample were measured using a HPGe detector (ORTEC GEM70200-p). This detector showed a relative efficiency of 55.2% at 1333 keV and an impressive energy resolution of 5.99 keV at 1333 keV (0.45%). To minimize background interference, 10 cm thick lead shields were employed to ensure low background counts (less than 5 cps) within a range of 60 keV to 3000 keV.

Calibration of the HPGe detector efficiency was meticulously carried out using standard gamma sources, including ¹⁵²Eu (24.5 kBq), ¹³⁷Cs (8.177 kBq), ⁵⁷Co (80.73 kBq), and ²⁴¹Am (6.516 kBq). The absolute efficiency (η) of the gamma source positioned at an identical distance from the HPGe detector was determined using Eq. (1). This rigorous calibration ensured the accurate and reliable measurement of the activity of the irradiated target. $\eta =\frac{N{F}_{\text{tsc}}}{A_0{e}^{-\lambda T}{I}_0{T}_{\text{c}}}$ (1) Here, N represents the photon peak counts obtained from the standard calibration gamma source, F_tsc denotes the correction factor for the coincidence summing effect, A₀ denotes the source activity at the factory, T is the time elapsed from the factory to the present, I₀ denotes the characteristic γ-ray transition relative intensity, and T_c is the counting time. To estimate the efficiencies corresponding to the γ-rays emitted from the decay of ⁵⁷Co, ¹³⁷Cs, ²⁴¹Am, ⁶⁰Co, and ¹⁵²Eu, a linear parametric model represented by Eq. (2) is employed.

The fitted curves of the interpolated and measured detector efficiencies are shown in Fig. 3. ϵ is the efficiency curve obtained from the experimental data, and ϵc is the correction efficiency for summing the coincidence effects. Furthermore, the correction for the summing coincidence effects was accomplished through a Geant4 simulation [29], to ensure accurate corrections and enhance the reliability of the calibration process. $ϵ=e^{a+b\left(\ln E\right)+c{\left(\ln E\right)}^2+d{\left(\ln E\right)}^3+e{\left(\ln E\right)}^4+f{\left(\ln E\right)}^5}$ (2)

Fig. 3Full-screen View

Fitting curves of the measured and interpolated detector efficiencies

Calculation of the γ-ray beam flux

The γ-ray beam flux ϕ(t) was determined using Eq. (3). $\varphi (t)=\frac{N_{\gamma }}{\sigma N_A{A}_b{I}_{\gamma }\eta f_{\text{t}}{f}_{\text{s}}}$ (3) Here, $N_{\gamma }$ is the effective count measured using the HPGe detector. NA is the number of target nuclei per unit surface, and Ab is the natural isotope abundance of the target. Iγ denotes the characteristic γ-ray transition relative to the target intensity. σ is the average cross section, where $\sigma ={\displaystyle \int }\sigma \left(E\right)n_{\gamma }\left(E\right)\text{d}E$. nγ(E) is the incident γ-ray beam distribution, calculated using the direct unfolding method and combined with the response function (R_f) of the LaBr₃(Ce) beam monitor simulated by the Geant4 code. The γ-ray spectrum (n_det) was measured by a LaBr₃(Ce) detector as detailed in [28] $n_{\text{det}}=R_f{n}_{\gamma }$ (4) The γ-ray beam energy distribution nγ can be deduced from Eq.4 via iterative least-squares fitting. We selected the total energy response of the LaBr₃(Ce) detector as the zeroth trial function obtained from Eq.4. Then, we iterated this procedure j times, yielding $n_{\det }{}^j=R_f{n}_{\gamma }{}^j$ (5) $n_{\gamma }{}^{j+1}=n_{\gamma }{}^j+\left(n_{\text{det}}-n_{\text{det}}{}^j\right).$ (6) Finally, nγ was obtained by iterating the program until convergence. The uncertainty of the unfolding method was less than 1%. The total efficiency of the LaBr₃(Ce) detector was approximately 84.75% to 87.15% at slant-scattering angles from 102 to 139. The time correction factor f_t is shown below. $f_{\text{t}}=\frac{\left(1-e^{-\lambda T_{\text{i}}}\right)e^{-\lambda T_{\text{w}}}\left(1-e^{-\lambda T_{\text{c}}}\right)}{\lambda }$ (7) Here, λ denotes the decay constant, T_i is the irradiation time, and T_w, called the cooling time, is the elapsed waiting time between the end of irradiation and start of the offline HPGe measurement count.

The self-attenuation coefficients (f_s) owing to the interactions of the γ-rays within the sample thickness are given by Eq. 8. μ is the attenuation coefficient obtained from NIST [30], and t is the mass thickness. $f_{\text{s}}=\frac{\mu t}{1-e^{-\mu t}}$ (8)

Target material for activation

The ¹⁹⁷Au(γ, n)¹⁹⁶Au and ⁶⁴Zn(γ, n)⁶³Zu reactions were specifically chosen to monitor the γ-ray beam flux at the SLEGS. The single-neutron separation energies for ¹⁹⁷Au and ⁶⁴Zn are 8.073 MeV and 11.86 MeV, respectively. Consequently, for these reactions, the γ-ray beam flux can be effectively monitored within the energy ranges of 8.07-21.00 MeV and 11.96-25.00 MeV, respectively, ensuring comprehensive coverage across the desired γ-ray beam energies. They exhibited a broader monitoring energy range, and the giant resonance excitation functions for these reactions are shown in Figs. 4(a) and (b). This presentation encompasses previously reported experimental data from the EXFOR experimental database and evaluated cross-sectional data from the ENDF/B-VIII.0 and IAEA/PD-2019 libraries. Owing to their substantial cross sections, these reactions facilitated short activation times, making them versatile for a variety of experiments. The half-lives of ^196gAu and ⁶³Zn were 6.1669 days and 38.47 minutes, respectively.

Fig. 4Full-screen View

(Color online) (a) ¹⁹⁷Au(γ, n)¹⁹⁶Au cross section as a function of the γ-ray energy from literature [32-40] and evaluated data (ENDF/B-VIII.0 and IAEA/PD-2019). (b) ⁶⁴Zn(γ, n)⁶³Zn cross section as a function of the γ-ray energy from literature [41-47] and evaluated data (ENDF/B-VIII.0 and IAEA/PD-2019) [48]

Fig. 5 shows the level scheme of ^196gAu and ⁶³Zn decays, along with the characteristic γ-ray energies and intensities associated with each. The relative nuclear spectroscopic data were sourced from the NuDat 3.0 database [31]. In addition to their utility in experimental settings, both reactions were well suited for offline measurements.

Fig. 5Full-screen View

(a) Typical energy spectra of an Au target irradiated by LCS γ-ray beams (Eγ=19.08 MeV). (b) Typical energy spectra of a Zn target irradiated by LCS γ-ray beams (Eγ=19.08 MeV)

The beam-flux activation monitor utilized a natural gold target (¹⁹⁷Au 100%) with a purity of 99.99% and thickness of 0.5 mm. In addition, natural Zn targets (⁶⁴Zn 49.2%, ⁶⁶Zn 27.7%, ⁶⁷Zn 4.0%, ⁶⁸Zn 18.5%, and ⁷⁰Zn 0.6%) with a purity of 99.99% and thickness of 2 mm were employed. The target had a diameter of 10 mm, which exceeded the diameter of the γ-ray beam restricted by a 3 mm coarse collimator. The target was strategically positioned on a multi-slot target holder along the beam axis and precisely placed in front of the experimental hall. The target underwent meticulous irradiation using a focused γ-ray beam. This deliberate irradiation resulted in well-controlled ¹⁹⁷Au(γ, n)¹⁹⁶Au and ⁶⁴Zn(γ, n)⁶³Zn reactions, which played a crucial role in the experimental procedure. The accuracy of cross-section data for nuclear reactions is crucial in beam monitoring. Experimental measurements of cross sections for the ¹⁹⁷Au(γ, n)^196gAu reaction [32–40] were conducted using γ-rays produced by several sources, including bremsstrahlung γ-rays [32, 40], positron annihilation in flight-generated quasi-monochromatic γ-rays [33, 34, 39], and LCS-generated quasi-monochromatic γ-rays [35–38]. The experimental cross-section data for the ⁶⁴Zn(γ,n)⁶³Zn reaction [41–47] were measured using monoenergetic γ-rays produced by nuclear reactions, including the ³H(p,γ)⁴He reaction [41, 47], the ⁷Li(p,γ)⁸Be reaction [45], and bremsstrahlung γ-rays [42–44, 46]. The IAEA provides the evaluated data for photonuclear reactions [48].

The ¹⁹⁷Au(γ, n)¹⁹⁶Au reaction produces unstable nuclei, such as ^196mAu and ^196gAu. Subsequently, ^196mAu is de-excited by emitting γ-rays, leading to the formation of ^196gAu. The decay of ^196gAu proceeds through either electron capture (93%), yielding ¹⁹⁶Pt, or through β^- decay (7%), resulting in ¹⁹⁶Hg. The decay profiles are shown in Fig. 5(a). Additional reaction details are summarized in Table 2.

Characteristic γ-ray de-excitation spectrum

The γ-ray beam flux was quantified by identifying the characteristic transition peaks associated with the ground state of ^196gAu, following the photon-neutron reaction with ¹⁹⁷Au. This ground state of ^196gAu has a half-life of 6.1669 days, making it a reliable marker for assessing the strength of the γ-ray beams. The irradiation, cooling, and counting times were carefully selected as t_i = 0.5637 days, T_w = 2.24, and T_c=224, 309 s, respectively. Notably, the cooling time exceeded two days, ensuring 99% decay of the excited state of ^196mAu (E_level=0.5957 MeV, T_1/2=9.6 hours) to reach the ground state. This meticulous time allocation enhanced the reliability and precision of the experimental measurements. Figure 5(a) shows the distinct characteristic the γ-ray transitions resulting from the irradiation of the ¹⁹⁷Au target using a 19.08 MeV γ-ray beam. Notably, the characteristic γ-rays of ^196gAu include peaks at 355.73 keV and 333.03 keV, originating from the β^- decay of ^196gAu, along with the peak at 426.10 keV, corresponding to the inner transition (IT) decay of ^196gAu. These features contribute to a comprehensive understanding of the experimental spectra.

⁶³Zn undergoes β⁺ decay, resulting in the emission of a characteristic peak at 511 keV because of the annihilation of positrons with electrons. The gamma-ray energy spectra recorded for zinc samples irradiated with 19.08 MeV photons are illustrated in Fig. 5(b). The experimental conditions included an irradiation time of t_i = 2 h, cooling period of T_w = 3.1 min, and counting time of T_c = 2 h. Notably, the statistical errors associated with these measurements were all below 1%, highlighting the precision of the experimental data.