title: Temporal signal stability of KCl:Eu(2+) storage phosphor dosimeters.
abstract: OBJECTIVE
  Current KCl:Eu(2+) prototype dosimeters require a wait time of 12 h between irradiation and dosimetric readout. Although irradiating the dosimeters in the evening and reading on the following day works well in the clinical schedule, reducing the wait time to few hours is desirable. The purposes of this work are to determine the origin of the unstable charge-storage centers and to determine if these centers respond to optical or thermal excitation prior to dosimetric readout.
  
  
  METHODS
  Pellet-style KCl:Eu(2+) dosimeters were fabricated in-house for this study. A 6 MV photon beam was used to irradiate the dosimeters. After x ray irradiation, dosimeters were subjected to external excitation with near-infrared (NIR) light, ultraviolet (UV) light, or thermal treatment. Photostimulated luminescence (PSL) signal's temporal stability was subsequently measured at room temperature over a few hours using a laboratory PSL readout system. The dosimeters were also placed in a cryostat to measure the temperature dependence of the temporal stability down to 10 K.
  
  
  RESULTS
  Strong F-band was present in the PSL stimulation spectrum, indicating that F-centers were the electron-storage centers in KCl:Eu(2+) where an electron was stored at a chlorine anion vacancy. Due to deep energy-depth (2.2 eV), F-centers were probably not responsible for the fast fading in the first a few hours post x ray irradiation. In addition, weak NIR bands were present. However, there was no change in PSL stabilization rate with intense NIR excitation, suggesting that the NIR bands played no role in the PSL fading. At temperatures lower than 77 K there was almost no signal fading with time. Noticeable PSL was observed for undoped KCl samples at room temperature, suggesting that Cl(2) (-) V(k) centers served as hole-storage centers for both undoped and doped KCl where a hole was trapped by a chlorine molecular ion. V(k) centers were stable at low temperature and became mobile at room temperature, probably causing the observed PSL fading with time. On the other hand, V(k) center could be stabilized by Eu(2+) activator or oxygen in the lattice, leading to the stable component in the PSL. A thermal process at elevated temperatures (60 °C or higher) was able to significantly accelerate the migration process resulting in a fast stabilization of PSL. However, this could not be accomplished using intense UV excitation.
  
  
  CONCLUSIONS
  Thermal treatment enables KCl:Eu(2+) prototypes to be ready for readout in 1 h without the need of applying a large time-dependent correction factor. However, this cannot be achieved using optical preexcitation.
authors: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO, USA.
authors: Zhiyan
authors: Z
authors: Xiao
authors: Rachael
authors: R
authors: Hansel
authors: Haijian
authors: H
authors: Chen
authors: Dongsu
authors: D
authors: Du
authors: Deshan
authors: D
authors: Yang
authors: H Harold
authors: HH
authors: Li
mesh_terms: D005063:Europium; D049449:Luminescence; D011189:Potassium Chloride; D011874:Radiometry / Q000379:methods*; D013696:Temperature; D013997:Time Factors
chemical_list: D005063:Europium; D011189:Potassium Chloride
references: Med Phys. 2009 Nov;36(11):5359-73
references: 19994544
references: Med Phys. 2003 Aug;30(8):2089-115
references: 12945975
references: Phys Rev B Condens Matter. 1991 Nov 1;44(17):9240-9247
references: 9998904
references: Med Phys. 2005 Sep;32(9):2987-99
references: 16266113
references: Med Phys. 2009 Jan;36(1):71-82
references: 19235375
references: Med Phys. 2010 Jan;37(1):146-53
references: 20175476
references: Med Phys. 2009 Aug;36(8):3748-57
references: 19746808
references: Med Phys. 2011 Aug;38(8):4681-8
references: 21928642
references: Phys Rev Lett. 1990 Nov 5;65(19):2438-2441
references: 10042548
references: Phys Rev B Condens Matter. 1996 Mar 1;53(9):5039-5042
references: 9984084
references: Med Phys. 2006 Nov;33(11):4064-72
references: 17153386
references: Med Phys. 2007 Dec;34(12):4594-604
references: 18196786
references: Phys Rev Lett. 1991 Dec 16;67(25):3571-3574
references: 10044769
references: Med Phys. 2011 Jul;38(7):4464-5; author reply 4466
references: 21859047