Nuclear Instruments and Methods in Physics Research A 648 (2011) S293–S296

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Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/nima

Ultrahigh-speed X-ray imaging of hypervelocity projectiles

Stuart Miller a,n, Bipin Singh a, Steven Cool a, Gerald Entine a, Larry Campbell b, Ron Bishel c, Rick Rushing c,Vivek V. Nagarkar a

a Radiation Monitoring Devices (RMD) Inc., 44 Hunt Street, Watertown, MA 02472, USAb ATA Range Operations Lead, Arnold AFB, TN 37389, USAc USAF, AEDC/DOT, 1099 Avenue C, Arnold AFB, TN 37389, USA

a r t i c l e i n f o

Available online 18 November 2010

Keywords:

High-speed X-ray imaging

Hypervelocity projectile

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.11.048

esponding author.

ail address: smiller@rmdinc.com (S. Miller).

a b s t r a c t

High-speed X-ray imaging is an extremely important modality for healthcare, industrial, military and

research applications such as medical computed tomography, non-destructive testing, imaging in-flight

projectiles, characterizing exploding ordnance, and analyzing ballistic impacts. We report on the

development of a modular, ultrahigh-speed, high-resolution digital X-ray imaging system with large

active imaging area and microsecond time resolution, capable of acquiring at a rate of up to 150,000

frames per second. The system is based on a high-resolution, high-efficiency, and fast-decay scintillator

screen optically coupled to an ultra-fast image-intensified CCD camera designed for ballistic impact

studies and hypervelocity projectile imaging. A specially designed multi-anode, high-fluence X-ray

source with 50 ns pulse duration provides a sequence of blur-free images of hypervelocity projectiles

traveling at speeds exceeding 8 km/s (18,000 miles/h). This paper will discuss the design, performance,

and high frame rate imaging capability of the system.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Since the discovery of X-rays by Wilhelm Roentgen in 1895 andthe rapid development and application of X-ray imaging that soonfollowed, many improvements have been made, the most recent ofwhich is the move to digital imaging systems using detectors suchas charged coupled devices (CCD), complementary metal-oxide-semiconductors (CMOS), and amorphous Si arrays (a-Si:H). Withthese digital imaging systems, high-speed real-time imaging hasbecome commonplace for optical systems and, to a certain extent,allowed ultra-fast X-ray imaging. Such systems have demonstratedtheir importance in defense applications such as impact analysisand flight analysis of hypervelocity projectiles. Many of thesestudies are performed using optical systems; however, even ultra-fast optical systems are limited in some cases due to impairedvisibility, such as in the presence of significant background light orwhere the camera view is obscured by the presence of a dust cloud.X-ray imaging, however, is well suited for such applications, andmany systems based on a storage phosphor and a single-shot X-raygenerator have been utilized. Such systems produce freeze-frameimages of remarkable quality, but are limited to a single image for agiven event or ongoing process, and thus critical informationregarding changing phenomena over short time periods is absent.Of real interest is ultrahigh-speed dynamic X-ray imaging, where a

ll rights reserved.

sequence of images is produced with microsecond inter-frametiming. Of course, dynamic imaging at such speeds is inherentlylight starved due to the brief acquisition times, and thereforerequires a special scintillator screen with intense light signal andrapid decay time, in addition to a powerful X-ray generator capableof delivering a rapid sequence of brief, intense bursts.

At RMD we have developed an ultra-fast high-speed X-rayimaging system that is capable of providing a 17-frame sequence ofimages as fast as 150,000 fps, although the first system imple-mentation was limited to capturing only 6 images in succession,due to limitations of the X-ray generator used. In its first deploy-ment, our ultrahigh-speed imaging system successfully imaged ahigh velocity projectile at Arnold Air Force Base (Arnold AFB, TN),capturing a series of 6 images. The system components required toaccomplish this are described in further detail here.

2. System components

2.1. Fast decay scintillator screen

Direct conversion materials such as amorphous selenium andmercuric iodide used in some X-ray imaging systems are not suitablefor this application because of their inherent lag and slow speed.Consequently, scintillator-based systems are the only feasible option.Ultrahigh-speed imaging is inherently light starved, which meansthat the scintillator must perform at the highest possible level in

S. Miller et al. / Nuclear Instruments and Methods in Physics Research A 648 (2011) S293–S296S294

terms of its X-ray absorption efficiency, light output, decay time (fastas possible) and afterglow. The speed at which the light decaysdetermines the interval at which images can be acquired (separated intime) without blurring.

At RMD we have developed a special high-speed scintillatorscreen (HSS-1) with an exceptionally high light signal and fastdecay, allowing imaging at up to 1 million frames per second. Fig. 1ashows the fast decay time of this scintillator material, which decaysexponentially, by 4 orders of magnitude, in only 5 ms. Also shown inthe figure is the decay of a GOS:Pr (Gadolinium Oxysulfide) screen,showing its exponential decay of 4 orders of magnitude in 30 ms.For imaging up to 50,000 fps the GOS:Pr scintillator material can beused; however, at higher frame rates, specially developed screenssuch as our HSS-1 with faster decay are required.

For the particular application described here, scintillatorscreens were designed for a large imaging area of 91.4�91.4 cm2

(3600 �3600), as shown in Fig. 1b. The screens were adhered to ¼00

plywood, which was then mounted to a solid steel frame. Theplywood provides a low Z and strong platform for mounting thescintillator. Each full-size screen was made by tiling four45.7�45.7 cm2 (1800 �1800) sections.

2.2. Pulsed X-ray source

The multi-anode flash X-ray source used in this system wasmanufactured by L3 Communications’ Pulse Sciences division(San Leandro, CA) and delivers high-intensity 50 ns X-ray pulses.

Fig. 1. (a) Scintillation decay plot of our HSS-1 scintillator screen, compared to that of a G

screen used in this study and in use at Arnold AFB.

Fig. 2. (a) Pulsed X-ray source, with 6 anodes in a 3�2 arrangement. (b) The Ultra

The very short duration of each pulse stops the motion of even thefastest of today’s projectiles. As long as the duration of the X-rays isshort enough that the object moves less than the equivalent of onepixel in the image, there will be no motion blur. With our system,the imaging geometry provides an effective pixel size of 1.79 mm,which means that we are safe from motion blur with this X-raysource at velocities even up to 35,000 m/s (78,000 miles/h). Themulti-flash system consists of six anodes, as shown in Fig. 2a, andeach anode can produce a single pulse during a high speed imagingsequence, when triggered by a TTL pulse.

Although the duration of the X-rays is very short, each pulse hasan intensity of 20 mR at a distance of 1 m. The intensity falls off toapproximately 4 mR at a distance of 2.1 m (7 ft) away from thesource, which was the distance to the scintillator screen in thedescribed application. This high-intensity X-ray pulse is convertedto optical light in the scintillator screen and produces images withgood signal-to-noise ratios (SNR).

2.3. High speed camera

The image-intensified Ultra 17 camera (shown in Fig. 2b) fromDRS Technologies (Parsippany, New Jersey) has 512�512 pixelresolution and can provide 17 images at a rate as fast as 150,000 fps(6.7 ms timing resolution). Our system uses standard F-mountNikon lenses allowing versatility in customizing for a specific setup.

The Ultra 17 uses a 2048�2048 pixel CCD fiber opticallycoupled to a GEN III image intensifier. The camera design allows

OS:Pr based screen, after one 50 ns X-ray pulse. (b) The large-area 91�91 cm2 HSS-1

17 high-speed camera from DRS Technologies, capable of 150,000 fps imaging.

Table 1Ultra 17 camera specifications.

Item Specifications

Intensifier GEN III GaAs intensifier with high

QE in 350–750 nm

CCD size 2�2 K pixels

Image size 512�512 K pixels

Imaging speed 6.7 ms/frame or �150,000 fps

Internal image storage capacity 17 images, stored at 150,000 fps rate

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the front-end sensitivity to be maximized, as there is no beamsplitter used, and thus each frame receives all of the available lightfrom the scintillation event. This, in turn, permits the camera to runat lower gain, which maintains improved image quality anddynamic range. To allow high-speed acquisition of images, theCCD is masked in such a way that 17 consecutive images can betaken from one image area on the sensor. The images are internallytransferred to adjacent on-CCD image storage areas after eachframe, and images are stored on-chip with all 17 frames read out bysoftware after acquisition. This arrangement allows imagingspeeds of 6.7 ms per frame, provides 512�512 pixel resolution,and allows us to capture 17 images in series. Table 1 lists the keyspecifications of the Ultra 17 camera.

Fig. 3. : This series of X-ray images of a projectile traveling at 3.0 km/s (6710 mph)

was acquired with our ultrahigh-speed dynamic X-ray imaging system. Note that

internal structures and materials within the projectile are also resolved. These are

raw images, without flat-field corrections applied.

3. Results and discussion

Our ultrahigh-speed imaging system was successfully demon-strated with a high velocity projectile in flight at Arnold Air ForceBase (Arnold AFB, TN), in their underground Range G facility. RangeG consists of a cylindrical, 10 ft diameter steel tank that isapproximately 1,000 feet long. The range can be evacuated tosimulate altitudes up to 220,000 feet. A large two-stage hydrogenlauncher can fire projectiles at velocities in excess of 7 km/s(15,658 mph) into the evacuated tank, allowing for a wide varietyof high velocity experiments to be conducted. This launcher, whichis the largest of its type in the world, can be adapted with bore sizesof 3.3, 4 and 8 in. This arrangement allows projectiles up to 20 kg tobe launched at hyper velocities for ballistic impact studies.

The series of digital X-ray images acquired with our system isshown in Fig. 3. This projectile was 6 in in diameter and 31 in. long,and traveled at a speed of 3.0 km/s (6710 mph). Six frames wereacquired at an imaging speed of 100,000 fps, a 10 ms frame rate, andan integration time of 5 ms was long enough to capture thescintillation light from the phosphor screen. As the images show,the image quality is quite high, and internal structures can also beresolved inside the projectile. The images shown here are the rawimages; however, flood-field correction can be done to removemuch of the fixed pattern noise seen here.

This system demonstrates that real-time, large-area, and ultra-high-speed X-ray imaging is now possible. With further develop-ments in increasing detector acquisition speed, it is anticipated thateven higher frame rates will be possible, up to 1 million fps. Asfaster and faster imaging rates are realized, new challenges arise asthe limitations of one or more of the system components arereached.

In the field of high-speed X-ray imaging, few systems are capableof ultra-fast data acquisition. In medical imaging, fluoroscopy systemsnormally acquire at a rate of 30–60 fps; however, special Cinemaximage intensifiers have been developed that utilize an imageintensifier [1] that can achieve a rate of 10,000 fps. Other high-speedsystems have been developed by RMD with high frame rates;however, these did not require large-area screens nor a scintillatorfor speeds greater than 100,000 fps [2–4].

Related stop-motion X-ray imaging systems can also provide veryuseful information, in cases where multi-frame imaging is not required.For example, recent advances in X-ray source technology has madepossible the introduction of very specialized pulsed X-ray sources, suchas the LINAC Coherent Light Source (LCLS) at SLAC National AcceleratorLaboratory (Menlo Park, CA; http://lcls.slac.stanford.edu). This X-raysource produces high-intensity 100 fs pulses that make it possible toexplore new frontiers in X-ray imaging, including the imaging of atomsand molecules. Previously, we reported on our development of a stop-motion X-ray imaging system for hypervelocity projectile locating andtracking [5]. The system that we described there was for large-areaimaging with single frame acquisition, with two separate, near-orthogonal views acquired simultaneously.

We expect that more applications will benefit from high-speed X-ray imaging systems such as we have developed and described in thepresent paper. In particular where high-resolution, fast-readout X-raydetectors are utilized, other applications would include medicalfunctional imaging, structural biology, microtomography, X-ray astron-omy, non-destructive testing, and basic physics research [6].

4. Conclusions

We have demonstrated a new state of the art in ultrahigh-speed,real-time, and dynamic X-ray imaging. The system we have

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designed, built, deployed and tested has successfully imaged amulti-frame sequence of an ultrahigh-velocity projectile travelingat 3 km/s, with the resulting images immediately available tosystem operators. This first system is currently in use at Arnold AirForce Base in Tennessee, where it will be used in ballistic impactstudies and related research.

Acknowledgments

Department of Defense SBIR contract # FA9101-04-C-0003.

References

[1] J. Honour, R. Hadland, Cinemax Image Intensifier for High Speed Cameras, SPIE832 (1987) 351.

[2] V.V. Nagarkar, S.V Tipnis, V. Gaysinskiy, S.R. Miller, I. Shestakova, Nucl. Instr. andMeth. B 213 (2004) 476.

[3] V.V. Nagarkar, S.V. Tipnis, V. Gaysinskiy, High Speed Direct X-ray ImagingSystem, Final Report, 3 Army Robert Morris ACQ, Contract no. DAAD05-01-C-0003, September 2001.

[4] V.V. Nagarkar, S.V Tipnis, T.K. Gupta, S.R. Miller, V. Gaysinskiy, M. Klugerman,M.R. Squillante, G. Entine, W.W. Moses, IEEE Trans. Nucl. Sci. NS 46 (3)(1999)232.

[5] Vivek V. Nagarkar, Bipin Singh, Stuart Miller, Larry Campbell, Ron Bishel,Rick Rushing, Proc. SPIE 6978 (2008) 697808.

[6] Huxley, et al., Biophys. J. Vol. 67 (1994).