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Aaron Walker
Aaron Walker

Alkali Free Download

MasterRoc SA 160 admixture is a high-performance alkali-free shotcrete accelerator for use in the wet-mix spraying process. It is a liquid additive whose dosage can be varied to the desired setting and hardening times.

Alkali Free Download

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We discovered and investigated sensitization of the luminescence of Tb3+ ions by Sn(II) oxocomplexes in highly activated alkali-free silicate glass. We show that the quantum yield of the luminescence sensitization in this glass can be increased by UV irradiation. The possibility for the use of such glasses as luminescing light filters for YAG-Nd laser quantrons is noted.

The generation of radio-frequency modulation in the polarisation or intensity of light transmitted through a precessing ensemble of optically pumped atomic spins is a phenomenon exploited in optical magnetometry1,2. The measured spin precession results from Zeeman splitting of the atomic ground state in the presence of a non-zero magnetic field. For alkali metal vapours, including commonly used isotopes such as \(^133\)Cs, \(^85\)Rb and \(^87\)Rb, Zeeman splitting is approximately linear in typical geophysical fields, and the precession frequency is related proportionally to the measured field by the gyromagnetic ratio \(\gamma \). Optical detection of the spin precession allows closure of a feedback loop, in which this oscillating signal is applied to the atomic vapour via modulation of the local magnetic field3, the rate of optical pumping4, or the polarisation of pump light5. Under the condition that the modulation phase matches that required for coherent feedback, the system undergoes a driven resonant response, strongly amplifying signals at the precession frequency. Such a system can be considered an alkali spin maser6.

Precessing geophysical magnetometers benefit from the atomic transduction of measured field to frequency, a physical property which can be determined precisely. Limiting noise sources on the precessing signal, such as photon shot noise, are white, meaning that the variance on this frequency (and hence magnetic field) measurement scales inversely with the cube of measurement time7 (for derivation see8, equations 1-17). Coupled with the high signal-to-noise measurement of optical rotation in the presence of a polarised optically pumped atomic sample, highly precise magnetometry may be achieved in geophysical magnetic fields9, with applications such as magneto-encephalography10 and GPS-denied navigation11. Furthermore, the direct mapping of precession frequency to magnetic field allows rapid changes in field to be resolved with high bandwidth, at rates even exceeding the precession frequency with use of signal phase estimation12. For alkali metals in geophysical magnetic fields, the magnetic precession (Larmor) frequency ranges between 70 kHz and 450 kHz. This frequency range is suitable for digitisation and real-time processing, which we exploit in this work.

In the case of a freely precessing magnetometer, the atomic coherence time \(T_2\) limits the duration of each measurement to \(\tau \simeq 2T_2\), fixing the minimum noise floor to this physical property of the atomic system. By contrast, a continuously oscillating signal may be sampled at a rate chosen to optimally cover the desired signal bandwidth and hence achieve the highest possible sensitivity to the desired signal source.

To achieve such a continuous atomic spin resonance, alkali spin masers have been realised using electronic feedback, using analogue phase shift and homodyne detection6. However, to reduce reliance on manually tuned analogue electronics, we use a continuous low-latency digital filter to drive resonant atomic spin precession in a \(^133\)Cs sample, implemented in firmware running on a field-programmable gate array (FPGA). In a practical sensor this represents a more scalable, flexible and portable option than analogue electronic feedback. This approach is particularly powerful in situations where the resonance phase condition is not known, for example in arbitrary geometries of the magnetic field or optical polarisation17. We implement a pair of matched finite-impulse response (FIR) filters, exploiting their fixed-phase output to generate the full analytic signal, from which a frequency-agnostic constant-phase-shifted signal can be generated and applied as magnetic feedback to drive the \(^133\)Cs spins. We describe the spontaneous emergence and amplification of resonant dynamics in this digital-atomic system, measure the signal-to-noise achieved in a compact sensor package, and characterise the system bandwidth and uniformity of response to rapid variation in the measured magnetic field.

The FIR filter architecture provides a flexible and powerful tool for defining the frequency-domain response of the system and allows a real-time estimator for the resonance frequency to be obtained. The use of a digital processor to modify and define spin maser feedback, governing and driving the inversion of Zeeman states, on a timescale faster than their precession is a valuable capability with potential for elaboration and further study. Interaction with alkali vapour ground-state Zeeman levels in geophysical fields offers a convenient sub-MHz frequency range for digital feedback and manipulation. In theses systems the use of well-established optical pumping techniques allows preparation of highly-polarised ensembles. The increasing processing power, availability and scalability of embedded processors will continuously offer enhanced impact in the manipulation of quantum systems and their application to portable devices. Development of this work has potential for study of modified spin dynamics, through infinite-impulse response (IIR) filtering, or the use of Bayesian estimation, such as Kalman filtering, to achieve CRLB-limited real-time frequency estimation.

The resolution of this digital spin maser system as a geophysical magnetometer can be estimated from the measured signal-to-noise-density (Fig. 3). Based on this we estimate the CRLB-limited root-mean-square resolution of this sensor to be 50 fT at 1 s sampling cadence. This measurement benefits from the persistent driven atomic oscillation, which allows \(\tau \) to be chosen on the basis of signal bandwidth, rather than limited by atomic coherence \(T_2\). The system also enjoys a 100% duty cycle, without down-time for cell heating or pump laser operation. The sensor hardware used is rather modest; a single VCSEL for optical pumping and probing and a mass-produced double-pass MEMS alkali vapour cell18. This is reflected in the signal-to-noise-density ratio observed. Several well-established techniques could be used to significantly enhance signal-to-noise, such as the use of a multi-pass vapour cell19 or the use of a re-pumping laser to achieve light-narrowing20.

Some concrete pavements commonly used at airports are susceptible to the destructive effects of alkali-silica reaction (ASR). The presence of ASR on concrete pavements can have a devastating effect on pavement performance, not only in terms of reduced functionality, but also in terms of shortened service lives.

1.1 This test method covers chemical determination of the potential reactivity of an aggregate with alkalies in portland-cement concrete as indicated by the amount of reaction during 24 h at 80 &#176 C between 1 N sodium hydroxide solution and aggregate that has been crushed and sieved to pass a 300-m sieve and be retained on a 150-m sieve.

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