Polarization enhancement technique for nuclear quadrupole resonance detection

Solid State Nuclear Magnetic Resonance ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

Solid State Nuclear Magnetic Resonance journal homepage: www.elsevier.com/locate/ssnmr

Polarization enhancement technique for nuclear quadrupole resonance detection Y.J. Kim n, T. Karaulanov 1, A.N. Matlashov, S. Newman, A. Urbaitis, P. Volegov, J. Yoder, M.A. Espy Los Alamos National Laboratory, P.O. Box 1663, MS-D454, Los Alamos, NM 87545, USA

art ic l e i nf o

a b s t r a c t

Article history: Received 15 March 2014 Received in revised form 29 April 2014

We demonstrate a dramatic increase in the signal-to-noise ratio (SNR) of a nuclear quadrupole resonance (NQR) signal by using a polarization enhancement technique. By first applying a static magnetic field to pre-polarize one spin subsystem of a material, and then allowing that net polarization to be transferred to the quadrupole subsystem, we increased the SNR of a sample of ammonium nitrate by one-order of magnitude. Published by Elsevier Inc.

Keywords: Nuclear quadrupole resonance (NQR) Nitrogen-14 Polarization enhancement NQR Ammonium nitrate

1. Introduction Nuclear quadrupole resonance (NQR) spectroscopy is a promising approach to detect solid explosives [1–3]. NQR is an analogous technique to nuclear magnetic resonance (NMR) in that both use radio frequency (RF) pulses to produce an oscillating magnetic field that is detected with a magnetometer. However, NQR has some distinguishing characteristics: (1) no external static magnetic field is required, although one can frequently be utilized to enhance the NQR signal, (2) the NQR resonance frequencies are determined by the surrounding chemical structures, which allows for NQR to distinguish between different materials, (3) for the common powder samples, there is no preferred orientation for the RF pulse, and (4) NQR typically has a low signal-to-noise ratio (SNR) due to the small energy level splitting, and is therefore harder to detect. A nucleus with spin 4 1=2 such as 14N, 35Cl, or 79Br additionally has an electric quadrupole moment (EQM) due to its non-spherical charge density. In NQR, the interaction of the EQM with an electric field gradient from the surrounding electronic structure results in quadrupole energy level splitting [3,4], in contrast to NMR where the Zeeman interaction provides the splitting. Because the quadrupole energy levels vary with the surrounding chemical structure, NQR is an outstanding method in identifying different substances.

n

Corresponding author. E-mail address: [email protected] (Y.J. Kim). 1 Present address: Senior Scientific, LLC, 800 Bradbury SE Suite 213, Albuquerque, NM 87106, USA.

Many solid explosives (TNT, RDX, HMX, or PETN) contain concentrations of the nearly 100% naturally abundant 14N nucleus with spin 1. In such a case, three quadrupole energy levels are found. The three transition frequencies between pairs of energy levels are given by

ν0 ¼

2 ωQ η ; 3




ν 7 ¼ ωQ 1 7

η 3

ð1Þ

where ωQ is the quadrupolar interaction and η is an asymmetry parameter related to the electric field gradient [3,4]. By convention, ν þ 4 ν  4 ν0 . For an axially symmetric electric field gradient η ¼ 0 and therefore only one transition frequency exists. In this paper, only the case of η a 0 is considered. Applying an RF magnetic pulse near a transition frequency creates an oscillation in the expectation value of the nuclear magnetic moment at the transition frequency, and this is observed as the NQR signal. Typical 14N NQR frequencies range from 0.4 MHz to 5 MHz [5], which is lower than the common NMR frequencies which can range from tens to hundreds of MHz. NQR signals are usually very weak due to the low resonance frequencies. In addition, for the common case of powder samples, each crystallite has a random direction with respect to the RF pulse, and therefore not all crystallites can be optimally excited. As a result, only 43% of the NQR signal from an optimally aligned single crystal is observable in a powder sample [6,7]. With these constraints, significant experimental efforts need to be employed to obtain a useful SNR in NQR. Our team has previously demonstrated the real-time screening of liquid explosives in an airport using our NMR based system,

http://dx.doi.org/10.1016/j.ssnmr.2014.05.002 0926-2040/Published by Elsevier Inc.

Please cite this article as: Y.J. Kim, et al., Solid State Nucl. Magn. Reson. (2014), http://dx.doi.org/10.1016/j.ssnmr.2014.05.002i

Y.J. Kim et al. / Solid State Nuclear Magnetic Resonance ∎ (∎∎∎∎) ∎∎∎–∎∎∎

2

MagViz [8,9]. In order to improve the explosives detection capabilities of this system, considerable effort has gone into incorporating an NQR system to handle solid explosives. In this paper, we describe the implementation of our NQR system by first evaluating its performance on sodium nitrite. We then demonstrate a polarization enhancement technique to increase the SNR of the NQR signal in ammonium nitrate. We are able to increase the SNR by a factor of 12, compared to the unpolarized experiment, allowing for accurate detection in roughly 40 s. While we focus here on explosives, this NQR approach could also be utilized screening for drugs [10,11] or chemicals such as heroin or cocaine.

ν þ ¼ 4:64 MHz is estimated to be 4  10  6 J=T m3 , which results in a net magnetic field strength of 5 pT. The predicted flux generated by this magnetic field in our solenoid was calculated to be 1:4  10  13 T m2 , which should induce a voltage of  300 nV in the coil, which is much larger than the thermal noise at 1 kHz bandwidth of  30 nV. The NQR probe was tuned to 4.64 MHz and matched at 50 Ω with a VIA Echo Analyzer by adjusting the variable capacitors producing a quality factor (Q) of 120. Fig. 2 shows the magnitude of an NQR spectrum from the sodium nitrite sample as the sum of 50 scans at room temperature. The pulse sequence used in this measurement was the inversionrecovery sequence

2. NQR system

ð257○ Þ  τ  ð119○ Þ  ACQ :

2.1. NQR hardware

The pulse sequence was chosen in order to observe the spin– lattice relaxation time (T1) [13]. In Fig. 2, we present the equilibrium NQR spectrum with τ of 300 ms. Here, 2571 and 1191 denote the flipping angle of the pulse, which corresponds to the 1801 and 901 pulses in NMR [7]. A simple phase cycling was used [13], with the 2571 and 1191 pulses cycling through the phases (x;  x) and (x, x), respectively. The duration of the first and second pulses was 185 μs and 86 μs, respectively, which required approximately 250 W. Due to the ringing time of  160 μs, data acquisition (ACQ) started 180 μs after the 1191 pulse. The pulse sequence was repeated 50 times, waiting approximately 400 ms between scans to allow the system to return to thermal equilibrium. As shown in Fig. 2, a clear NQR peak was detected at 4.6403 MHz. By changing the sample temperature from 20 1C to 29 1C, the temperature dependence of the NQR frequency was found to be ð1:9 7 0:1Þ kHz=1C. This agrees well with that in Ref. [14]. The NQR system was also able to measure relaxation times, and we observed the T1 and the spin–spin relaxation time ðT 2 Þ to be ð88 7 2Þ ms and ð4:6 7 0:2Þ ms, respectively, which are in agreement with those in Refs. [15,16]. Based on these NQR measurements in sodium nitrite, we concluded that our NQR system performed well and was ready for further measurements.

We constructed an NQR system designed to measure NQR frequencies ranging from 0.4 to 5 MHz at room temperature. It consists of a Tecmag APOLLOTM console, to generate the RF pulse sequence and acquire the NQR signal; an Amplifier Research 1000LP power amplifier; a homemade portable NQR probe; and a preamplifier. In order to obtain the highest possible SNR, we designed an ultra-low-noise voltage amplifier module with an AC coupled FET input in cooperation with Physikalisch-Technische Bundesanstalt (PTB) and Magnicon GmbH. The preamplifier has a bandwidth from 1 Hz to 5 MHz, pffiffiffiffiffiffiand a voltage gain of 1000. Its input voltage noise is 0:5 nV= Hz at the kHz frequency range. As shown in Fig. 1, our NQR probe is composed of a solenoid, variable tuning and matching capacitors, and supporting circuitry, including a quarter-wave circuit and cross-diodes, in a standard design [12]. The solenoid (diameter of 3.5 cm, height of 7 cm) is made up of 40 turns of 21 AWG copper wire and serves as both the pulse transmitter and the signal receiver. A plastic 60 ml sample container is placed inside the solenoid. A Schottky diode (1N5817) was used for the cross-diodes. For RF shielding, the NQR probe is enclosed by an aluminum box with dimensions of L30 cm  W18 cm  H13 cm. 2.2. NQR signal in sodium nitrite We used a 40 g powder sample of sodium nitrite (NaNO2) to validate the performance of our NQR system. Sodium nitrite was chosen because its relatively high NQR frequencies, of a few MHz, and short spin-lattice relaxation time make detection relatively easy. The room temperature RF-induced bulk magnetization at

ð2Þ

3. Polarization enhancement 3.1. Background Enhancement of SNR is practicable by employing a polarization enhancement technique, a standard method used in NMR [17]. In NQR, a pre-polarization static magnetic field is used to polarize some other spin system than the target NQR system, typically a spin 1/2 1H subsystem. The energy splitting of the protons is much larger than that of nitrogen. Proton polarization transfers to quadrupolar systems when the Larmor frequency and the NQR frequency equalize during demagnetization through the 1.1

NQR Intensity (a.u.)

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

Fig. 1. An internal photograph of our homemade NQR probe. The regions A, B, and C respectively show the sample containing solenoid; the variable tuning and matching capacitors; and the supporting circuitry, with a quarter-wave circuit and a cross-diode. An aluminum enclosure houses all the components to shield from RF noise. The input from the power amplifier and the output to the preamplifier are both located on the right side.

0.1 0 4.632

4.634

4.636

4.638

4.64

4.642

4.644

4.646

Frequency (Hz)

4.648 x 10

Fig. 2. The NQR spectrum from 40 g of sodium nitrite powder after 50 data averages. A clear NQR peak was detected at 4.6403 MHz.

Please cite this article as: Y.J. Kim, et al., Solid State Nucl. Magn. Reson. (2014), http://dx.doi.org/10.1016/j.ssnmr.2014.05.002i

Y.J. Kim et al. / Solid State Nuclear Magnetic Resonance ∎ (∎∎∎∎) ∎∎∎–∎∎∎

mechanism of cross-polarization [18–23], increasing the NQR signal. In this technique, the homogeneity of the polarization field is immaterial, because enhancement of SNR is determined by the average magnetic field [20]. This makes the process very promising for practical applications where generation of a uniform field may not be possible. Polarization enhancement becomes effective for a material with low SNR (e.g., less than one). The poor SNR is generally due to low NQR frequencies, small sizes, short T2, or broad line width. A practical consideration for polarization enhancement is that the higher the NQR frequency the larger the magnetic field required to achieve any benefit. In such a case NQR detection requires multiple scans and correspondingly long measurement times. When T1 is a few tens of seconds, the required measurement time could be hours. The polarization enhancement method could considerably reduce the measurement times by enabling NQR signals to be detected with a single pulse sequence.

3

3.3. Experimental setup We used an electromagnet (WALKER Scientific Inc. HF-7H), controlled with a power-amplifier (AE Techron LVC5050), to prepolarize the protons in the ammonium nitrate powder,  1 m away from the NQR probe. This could generate up to 280 mT with our configuration. As shown in Fig. 4, we programmed the electromagnet to (1) ramp up the static field in 2 s, (2) hold the static field constant to pre-polarize the protons, (3) decrease the field in 1 s, (4) remain at zero field while the sample is moved from the magnet before the NQR sequence is applied. In the actual experiment, the ammonium nitrate powder is first located in the center of the electromagnet for pre-polarization. After demagnetization, during which cross-polarization between 14 N in the NO3 ions and 1H in the NH4þ ions occurs, the sample was quickly, manually moved through an aluminum wave guide tube and into the NQR probe. This method was chosen because it avoids the complexity of having a large coil around the NQR probe to generate the static field. With the sample in the probe, the multi-pulse sequence in Eq. (3) was then applied.

3.2. Choice of sample

NQR Intensity (a.u.)

The proton system in the ammonium nitrate powder was prepolarized for 30 s at 280 mT. After demagnetization, the CPMG sequence in Eq. (3) was applied only once to obtain an NQR signal. The total measurement time, including the polarization, was  40 s. This was significantly less than the 1.5 h measurement 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 3.8

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7 x 10

2s

Sample moving

Static field

Cross polarization

Fig. 3. The NQR spectrum from 50 g of ammonium nitrate powder after 1.5 h. We have subtracted the signals from measurements without the sample, to eliminate unwanted noise peaks. A NQR peak 423.2 kHz is clearly detected, with the SNR after 1.5 h being  27.

ð3Þ

with n ¼800 and τ1 ¼ 1:3 ms required by the long ringing time of about 1.2 ms. Here, 2τ1 ¼ ðτ2 ACQ  τ3 Þ. The CPMG was repeated 100 times with a delay time of 50 s between scans. This resulted in a total measurement time of around 1.5 h. After subtracting identical measurements made without the sample, to remove noise peaks, the resulting signal is shown in Fig. 3. A clear NQR peak is observed at 423.2 kHz with an SNR of  27. This measurement is quite good detection in terms of an SNR of  2.7 for one scan. Nonetheless, in an attempt to enhance the detectability for smaller quantities of material in a reasonable time, we attempted polarization enhancement, as we discuss in the next section.

3.9

Frequency (Hz)

Pulse sequence

ð1191Þx  τ 1 ðð2571Þy  τ2  ACQ  τ3 Þn

3.4. Results in ammonium nitrate

Field

Ammonium nitrate (NH4NO3) was chosen to demonstrate the polarization enhancement technique for several reasons [23]. It is of significant interest to the explosives detection community because of its use in improvised bombs such as the Oklahoma City bombing in 1995, and because it is the main component of ANFO (ammonium nitrate/fuel oil), a common explosive. Additionally, it contains two spin subsystems, 14N and 1H nuclei, making polarization enhancement possible. Ammonium nitrate is also a very hard substance to detect. It has two non-equivalent 14N sites, which reduces the number of nuclei that can be excited by a single frequency, and its NQR frequencies are less than 500 kHz [24,25]. Moreover, its T1 of 16 s at room temperature requires long data averaging times. Thus, traditional NQR detection is quite challenging and ammonium nitrate is an excellent candidate for the polarization enhanced NQR detection. First, we measured an NQR signal in 50 g ammonium nitrate powder without using the polarization enhancement technique. The NQR frequency of ν  ¼ 423 kHz (associated with the NO3 ion) was used in all measurements because it is less sensitive to temperature changes than ν þ ¼ 498 kHz. Our NQR probe was tuned to 423 kHz and matched at 50 Ω with a VNA2180 vector analyzer. For this measurement, we replaced the 40 turns solenoid used for sodium nitrite with one having 76 turns. This was done to compensate for the loss in Q factor at the lower frequency and give acceptable values of the tuning and matching capacitors, and resulted in a new Q factor around 80. For more efficient detection, the multi-pulse sequence Carr–Purcell–Meiboom–Gill (CPMG) sequence was used (in the NQR literature, this is functionally referred to as the spin-lock-spin-echo (SLSE) pulse sequence [26]). This is represented as

Detection of NQR

1s 2s

Time π 2 π π π π

… Time

Fig. 4. Polarization enhancement technique. The pre-polarization of the proton system in ammonium nitrate is followed by demagnetization for cross-polarization. After demagnetization, the material is quickly moved inside the probe to detect the NQR signal.

Please cite this article as: Y.J. Kim, et al., Solid State Nucl. Magn. Reson. (2014), http://dx.doi.org/10.1016/j.ssnmr.2014.05.002i

Y.J. Kim et al. / Solid State Nuclear Magnetic Resonance ∎ (∎∎∎∎) ∎∎∎–∎∎∎

NQR Intensity (a.u.)

4 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 3.8

3.9

4

4.1

4.2

4.3

Frequency (Hz)

4.4

4.5

4.6

4.7 5

x 10

Fig. 5. The polarization enhanced NQR signal at 423.5 kHz in ammonium nitrate. The total measurement time including polarization was around 40 s, with the SNR being  23.

Furthermore, we examined the polarization time dependence of the NQR signal. For this measurement, the pre-polarization field was fixed at 280 mT while the polarization time was varied. The result is shown in Fig. 6(b) and reveals that the NQR signal increases exponentially with the pre-polarization time. Again, the error bars in the plot are from four repeated measurements. We fit the data with a single exponential model (y ¼ ae  x=t1 þ y0 , where a ¼ ð  0:61 70:03Þ, y0 ¼ ð0:98 70:01Þ, and t 1 ¼ ð11 7 2Þ). This result provides the optimal duration of the polarization time to improve SNR. It confirms that the 30 s polarization time we used is long enough to obtain almost the maximum NQR signal.

4. Conclusion We demonstrated that polarization enhanced NQR in powdered ammonium nitrate could result in 10-fold improvement in the SNR. This justifies incorporating our NQR system with our existing NMR system, where the pulsed NMR fields can be utilized for polarization enhanced NQR. This will enable both liquid and solid explosive near-simultaneous detection in the near future. The combination would be a unique capability for global security.

Acknowledgments The authors are grateful for helpful discussions with Dr. Michael Malone. This work was supported by the Los Alamos National Laboratory LDRD office through Grant 201202187ER. References

Fig. 6. (a) The measurement of polarization enhancement factor as a function of polarization field strength. The error bars are from four repeated measurements. The solid curve indicates a linear fit. The polarization enhancement technique improved the SNR by one-order of magnitude at 280 mT. (b) The NQR signal as a function of polarization time. The solid curve indicates an exponential fit.

without polarization enhancement that produced a comparable SNR. Fig. 5 shows the polarization enhanced NQR signal, clearly measured at 423.5 kHz with an SNR of  23. We investigated the dependence of the NQR signal on the polarization field by varying it from 20 mT to 280 mT, with the result shown in Fig. 6(a). The NQR signal intensity increases linearly with the pre-polarization field up to our experimental limit of 280 mT. This is as expected because the proton polarization should also increase linearly with field in the high temperature limit. We extrapolated the NQR signal at no pre-polarization field with a linear fit (model : y ¼ ax þ b, where a ¼ ð1:00 7 0:05Þ, and b ¼ ð4:1370:03Þ  10  2 ) to estimate the polarization enhancement factor. The largest enhancement factor we observed, at 280 mT, was 12.570.6. This demonstrates that our polarization enhancement technique enhanced the NQR signal by one-order of magnitude.

[1] V.S. Grechishkin, N.Y. Sinyavskii, Physics-Uspekhi 40 (1997) 393–406. [2] J.B. Miller, G.A. Barrall, Am. Sci. 93 (2005) 50–57. [3] J. Fraissard, O. Lapina, Explosives Detection Using Magnetic and Nuclear Resonance Techniques, Springer, Dordrecht, Netherlands, 2009. [4] J.A.S. Smith, J. Chem. Educ. 48 (1971) 39–49. [5] A.N. Garroway, M.L. Buess, J.B. Miller, B.H. Suits, A.D. Hibbs, G.A. Barrall, R. Matthews, L.J. Burnett, IEEE Trans. Geosci. Remote Sens. 39 (2001) 1108–1118. [6] Y. Lee, Concepts Magn. Reson. 14 (2002) 155–171. [7] S. Vega, J. Chem. Phys. 61 (1974) 1093–1100. [8] M. Espy, M. Flynn, J. Gomez, C. Hanson, R. Kraus, P. Magnelind, K. Maskaly, A. Matlashov, S. Newman, T. Owens, M. Peters, H. Sandin, I. Savukov, L. Schultz, A. Urbaitis, P. Volegov, V. Zotev, Supercond. Sci. Technol. 23 (2010) 034023. [9] M. Espy, S. Baguisa, D. Dunkerley, P. Magnelind, A. Matlashov, T. Owens, H. Sandin, I. Savukov, L. Schultz, A. Urbaitis, P. Volegov, IEEE Trans. Appl. Supercond. 21 (2011) 530–533. [10] R. Blinc, J. Seliger, A. Zidansek, V. Zagar, F. Milia, H. Robert, Solid State Nucl. Magn. Reson. 30 (2006) 61–68. [11] J. Barras, D. Murnane, K. Althoefer, S. Assi, M.D. Rowe, I. Poplett, G. Kyriakidou, J.A.S. Smith, Anal. Chem. 84 (2013) 2746–2753. [12] N. Hiblot, B. Cordier, M. Ferrari, A. Retournard, D. Grandclaude, J. Bedet, S. Leclerc, D. Canet, C. R. Chim. 11 (2008) 568. [13] M. Ferrari, D. Canet, Mol. Phys. 107 (2009) 2419–2430. [14] T. Oja, R.A. Marino, P.J. Bray, Phys. Lett. 26A (1967) 11–12. [15] A.N. Garroway, M.L. Buess, J.P. Yesinowski, J.B. Miller, SPIE Subst. Detect. Syst. 2092 (1993) 318–327. [16] G. Petersen, P.J. Bray, J. Chem. Phys. 64 (1976) 522. [17] S.R. Hartmann, E.L. Hahn, Phys. Rev. 128 (1962) 2042–2053. [18] T.N. Rudakov, P.A. Hayes, J. Magn. Reson. 183 (2006) 96–101. [19] J. Luznik, J. Pirnat, Z. Trontelj, Solid State Commun. 121 (2002) 653–656. [20] J. Luznik, J. Pirnat, V. Jazbinsek, T. Apih, R. Blinc, J. Seliger, Z. Trontelj, J. Appl. Phys. 102 (2007) 084903. [21] K.R. Thurber, K.L. Sauer, M.L. Buess, C.A. Klug, J.B. Miller, J. Magn. Reson. 177 (2005) 118–128. [22] R. Blinc, T. Apih, J. Seliger, Appl. Magn. Reson. 25 (2004) 523. [23] D.W. Prescott, M.W. Malone, S.P. Douglass, K.L. Sauer, J. Chem. Phys. 137 (2012) 214201. [24] J. Barras, M.J. Gaskell, N. Hunt, R.I. Jenkinson, K.R. Mann, D. Pedder, G. N. Shilstone, J.A. Smith, Appl. Magn. Reson. 25 (2004) 411–437. [25] T.N. Rudakov, Appl. Magn. Reson. 43 (2012) 557–566. [26] J.B. Miller, Counterterrorist Detection Techniques of Explosives, Elsevier, Oxford, UK, 2007, pp. 157–198.

Please cite this article as: Y.J. Kim, et al., Solid State Nucl. Magn. Reson. (2014), http://dx.doi.org/10.1016/j.ssnmr.2014.05.002i